Are you serious...? So I have to rescript every dialog in the game? @_@
Its a matter of quest conditions, not dialogue conditions
Most of the 'generic' stuff NPCs say is in quests aptly named Generic, which is basically the core of the dialogue system and has all the topics NPCs say when hit, robbed, etc... and many Generic(name of the race). There you have all the basic topics that make conversations between NPCs possible: HELLO, GREETING and GOODBYE are probably the most important ones since conversations cannot 'fire' without them (their nature seems almost hardcoded).
And like most other quests, the Generic quests have the IsPlayableRace condition. Conditions on the main quest field are simply what conditions are applied to ALL dialogue under that quest (you can't have dialogue under no quest, or it will simply not appear anywhere)
Why this condition? Because Beth made DAMN sure you would find no unvoiced dialogue in the game. Unplayable races like dremora, saints and seducers have their own generic dialogue quests with conditions restricted to their races, and their dialogue lacks many topics found for the other races (since dremora don't have to chat about mudcrabs...)
I'm sorry to say that the only practical solution is for those races to remain playable. Sure, your char gen screen will be a bit cluttered, but that is a small price to pay if you ask me. Put a description to them like DO NOT USE to prevent users.
OR if you don't mind using OBSE, try using SetRacePlayable (your race) 0 while in MenuMode 1036, and after you are out of the Chargen menu, SetRacePlayable (your race) 1 to ensure dialogue plays on them normally.