Sunflower lecithin (sorry, best link I could find) is gaining in popularity as an alternative to soy lecithin because it is widely perceived to have a neutral taste and actually has superior emulsifying properties. It's a little on the expensive side, though.
Soy lecithin by itself doesn't taste horrible if you buy it as a food additive (as opposed to a nutritional supplement). It's common to find in a lot of popular frozen pasta-and-sauce dishes, such as Michelina's, which obviously must be microwaveable without the sauce curdling or separating.
Another option is Sodium Stearoyl Lactylate, if you can get hold of it. This one adds a little sweetness which is generally pleasant-tasting, or at least inoffensive. You'll find it used most often as a dough conditioner or in baked goods, but another popular use for it is in commercial sauces, although unfortunately I can't find specific examples right now.
But probably the best emulsifier for what you're trying to do here is xanthan gum. It's also used as a gelling agent, but in small quantities works as just an emulsifier. You can find it, for example, in this Bertolli Creamy Alfredo Pasta Sauce - exactly the sort of thing you're making. It's also used in the Stouffer's Alfredo sauce.
Mix in a little lecithin or xanthan gum as an emulsifier while the sauce is fresh or cooking and it's very unlikely that your sauce will separate, either in the fridge or in the microwave.
As for what you can do to specifically prevent separation while reheating, if you didn't add any emulsifiers in the first place... I really don't think there's anything you can do. If you're a slow enough eater the sauce will eventually separate right in front of you; that's just what happens when you have water and fat in the same sauce. Best to reheat on the stove top in that case, and stir frequently to prevent any further separation.
We used to have fried spaghetti for leftovers growing up. Take cooked spaghetti in sauce and put it in a warm frying pan with a little oil.
I've also had noodle fritters with leftover spaghetti, take undressed spaghetti and dip in a batter, then pan or deep fry. Somewhat like a potato pancake, we would eat them with sour cream.
Best Answer
If you're going to reheat pasta, microwaving is a good way. Adding a few drops of water and covering helps the texture if it doesn't have sauce already on it.
You can actually do something close to a stir fry - heat some oil and use it to cook tasty ingredients (onions, garlic, herbs, spices) and then stir in the pasta (and things like olives or sweetcorn that might want heating but don't need cooking) over a gentle heat until hot enough to serve.
Grilling is less good, but still not impossible for some dishes. For bare pasta it will dry out it, but immerse in sauce and top with cheese and you can get something nice. Strictly speaking the grill doesn't do the reheating, the (hot) sauce does - a grill would burn the top before the bottom is hot.