Some sort of nutty flavor perhaps? Hazelnut syrup? I think it should go well with both chocolate and orange. Otherwise I think the cake will be quite delicious even without a substitute, just using plain buttercream.
EDIT: I just noticed that the cake has an almond base. Perhaps a few drops of Amaretto in the buttercream would work. That would play nicely with the almond flavor already there.
If you're looking to parboil, likely your best bet is one that you've already dismissed -- inedible casings that you'd remove after cooking. You might even be able to get away with clingfilm, parchment paper or non-stick aluminium foil.
If you really want an edible casing, they do exist, just enter 'vegetarian sausage casing' into your preferred internet search engine, and you should find them. (I assume they're all reselling the same product, as the only option I've found is for 15/16", ~10lb strand)
As you also mention that part of your issue is not wanting to eat intestines, there are also collagen casings which are more readily available, but aren't vegetarian.
update:
To deal with the issue of packing the sausage so you don't have air bubbles :
Place a line of meat down the center of what you're using, roll it up tightly, then seal the ends.
To get the meat lined up tightly: fold the back end of the sheet over the front, hold down the edges and use the edge of a sheet pan to press the meat tightly towards the back. If you're using something that might tear easily (eg, aluminum foil), you can work on top of another sheet of paper (waxed, butcher, freezer, etc.) and fold it over before pushing back, but don't roll it up to make the final sausage.
Best Answer
Sweet pickles, miniature if you're not dicing them, it doesn't matter if you are dicing them. Look for a bit of crunch; cornichons provide texture as well as flavor.