I think the problem is actually not the heating, but the refrigeration!
Consider: bacon fat solidifies easily and thickly, and does so even at room temperature. When you throw it in the fridge, the micro-droplets of bacon fat will turn to solids and clump together. These solid droplets are frozen in place when solid, but when you thaw it, they melt and reveal how wrecked your emulsion is. Now, you could add additional emulsifying agents (lecithin, extra yolks) but that's not really going to solve the problem of refrigeration.
The solution is to break up the bacon fat as it melts, and re-establish the emulsion before it can break. To do this, you have to whisk constantly as you gently heat the mayonnaise, generally in a warm water bath.
How to execute this in a restaurant setting:
Prep a big batch of bacon-mayo and throw it in the fridge. Just before service, warm some water in a pot and throw a cup of the mayo in a small bain marie or metal 6th hotel pan. Immerse the bain/pan in the pot, and whisk it as it melts. Hold the mayonnaise for service in water warm enough to melt bacon fat, and DISCARD THE WARM MAYONNAISE EVERY TWO HOURS AND THAW A FRESH BATCH. Make sure no cooks get lazy about that -- it's a food safety problem. Ideally, you should be using pasteurized eggs to reduce the risk of salmonella. Basically, you're treating your mayonnaise like a Hollandaise or Bearnaise.
I've found that the thawing trick works fairly well for home hollandaise (which most people say shouldn't refrigerate), and which does the same thing if heated fast. It should apply to easily-broken mayonnaise too.
Starting from the basics: Mayonnaise, as you know, is a combination of water-based liquids, water-soluble ingredients, and lipids (fats/oils). Since water and lipids are immiscible, that makes mayonnaise an emulsion.
Because the droplets (of fat) suspended in an emulsion are not actually dissolved, the properties of that emulsion depend entirely on the size of those droplets and their dispersion. The most likely reason that your mayonnaise tasted like oil is that it actually was pure oil in spots.
The technical term for this is flocculation.
(source: Cube Cola)
This is probably what happened to you - it's possible that if you had really poor dispersion, you might have even been closer to the "coalescence" stage.
To use a more tangible example, consider what happens when you dissolve flour or corn starch in cold water, then heat it. The starch gelatinizes and you end up with a fairly uniform, thick paste. Now think of what happens if you toss it into hot water; you'll tend to end up with something that isn't uniform, instead you'll end up with big globs of cooked flour floating around in thin, cloudy water.
Keep in mind that the chemistry is completely different with an emulsion - in fact, there technically is no chemistry happening with an emulsion until emulsifiers come into the picture - but the concept is the same. You might not be able to see those globs of oil floating around in the water as well as you can see the globs of flour, but if you didn't get proper dispersion and suspension, they're there, and they will taste exactly how you'd expect a glob of pure oil to taste.
Traditional mayonnaise uses raw egg yolk (containing lecithin) and mustard (containing mucilage), both of which act as emulsifiers. These are called "emulsifiers" mainly because they help the emulsion to stay stable, which is why the store-bought mayonnaise doesn't separate (it also probably has a few extra additives). However, they aren't all that helpful for getting that initial dispersion; the most efficient way to do that is to let small drops of oil into a liquid that is being constantly and uniformly agitated.
You can do this by stirring, but an even better way is to use an immersion blender with emulsifying blade. Note that this is not the flat aerating blade that is often confused with the emulsifying blade, nor is it the star-shaped liquefying blade that is the default on most sticks and many manufacturers confusingly call an "emulsifying" blade. The one you want looks a bit like a hubcap; it's flat with several slits or holes and is sometimes also called a "smoothie blade" or "whisk blade":
or
(the one I'm talking about is the bottom left)
These things are perfect for preparations like mayonnaise, but if you don't have one, you can get halfway decent results with a wire whisk. You'll just need to use a lot of elbow grease.
If you get really good dispersion, and use sufficient emulsifiers such that the emulsion doesn't separate too fast, then I promise you, your mayo won't have that "fatty" taste and it will be 1000 times better than the store-bought goop.
Best Answer
Regarding Romanian recipe...
Actually in Romania people tend not to use raw yolks so much. Most often we eat relatively raw yolks just in fried eggs or soft boiled eggs. In most of recipes the yolks are cooked.
Regarding mayonnaise there are three ways of preparing it: using just raw yolks (most simple), using raw and cooked yolks 50-50, and using just cooked yolks (harder).
For the last one, which helps storing mayonnaise safer and longer, the eggs are well boiled and then the yolk is removed. After cooling done until lukewarm it is finely crushed (often by passing it through a sieve several times) and then oil is added little by little. Sometime a small quantity of soft mustard is added for creating more chance of success. For one yolk about 100 ml oil can be incorporated (depending yolk size, as well). The mayo resulting it is slight harder then regular one. For making it softer, if you would like, you can add at last bubbled water (like a teaspoon), some lemon juice and even a tablespoon of cream or yogurt. Enjoy! Pofta buna!!
PS: we have some recipes of mayo without yolks, at all. These are for the period when we traditionally fast and no animal product is allowed.