The thing is, Devils by their nature would have to be sneaky to infiltrate the FR. Faerun has not only all the good groups working out there, but many evil groups, such as Bane, Cyric, Talos, etc. would work against them too, fighting what they would perceive as a threat to their power.
Balzebul would work in anywhere - he's the Lord of Lies in the infernal pantheon.
Mephistopholes would come in anywhere that a mage might be bargaining for more power. Imagine someone like old Meph' who made it widely known that those who bargained with him would gain sorcerer power in exchange for servitude. How many failed wizards with low INT and WIS would accept an offer to gain both CHA and levels in sorcerer in order to gain power they normally could not wield otherwise?
Back when the Ruins of Ascore was active, I believe there was a pit fiend who ruled over that area, correct? If so, then Asmodeus would have a direct line into wherever a pit fiend extended his power.
Finally, many devils can be introduced into a campaign under the guise of the best of intentions. Imagine a group who had a mentor who gave them magic items, timely info, etc. to help them out - yet it turned out that all of the info they acted on directly caused someone else importance in the Realms to suffer. If the PC's took everything at face value, you could have a devil behind the scenes manipulating things to make the PC's patsies to a great work of evil - imagine the players' faces when they find out that all the work they did actually helped demonkind! (For that matter, check out Piratecat's story hour now for a scenario where the heroes are working to "fix" a problem that they sort of made worse by taking out the lesser evil.
