So I love the mod Mannimarco Revisited, but I hate that it requires that a new playable race exist in order for the new voice/dialog to work right... Is there anything that could be done to get rid of that race or make it unplayable, that would still allow Mannimarco to have the new custom sound files?

So what happens when you open up the Construction set and just uncheck "playable" in the race definition?

Dremora, Mazken, Aureals (and Sheogorath) are unplayable and have voices, so there's no need for a race to be playable, just to have it speak.

Mannimarco is a vanilla character, that has normal dialogue. Making him an unplayable race probably cuts him off from that dialogue, since Dremoras, Mazken, Aureals and such all have their own custom dialogue and apparently can't use normal dialogue.

Might be able to redo all his dialogue to work, but risks incompatibility.

Might also be able to make an OBSE plugin that starts with him unplayable and uses "SetRacePlayable" after the CharGen is done, or which toggles his race to unplayable every time the reace menu is opened for any reason.