Both my D&D lore and my google-fu are failing me. Do any of the known demon lords have established relationships with orcs? Obviously I need to know for my tribe of demon-worshipping orcs.
In FR, orcs have been associated with tanar'ri in the past. Tanarukks were orcs whose blood had been tainted by an evil outsider, in many cases a tanar'ri demon. They originated as orc-demon crossbreeds in the region around Hellgate Keep.
Pretty much Orcs tend to focus on the Orc God Gruumsh and others of the Orc Pantheon. If you want them to worship a Demon Lord there are a lot of choices, but I can't think of any that really work for the Orcs. It would probably be easier to say that Gruumsh has some Demon Servants.
Baphomet would probably work the best outside of using Gruumsh. Orcus despite the name probably would not be the best what with the entire hates life thing and everything should become undead.
It's kind of academic, no? Evil God vs. Evil Demon Lord only really matters if you care about the "lore" on a deep level anyway, so for most games, who is to say that Gruumsh isn't a Demon Lord? As far as the PC's and the world is concerned, would that fine distinction really matter?