Dirge Singer:
- Any Neutral Alignment
- Ability to Turn/Rebuke Undead or ability to Summon/Control undead via spells
- Perform (Sing): 8 Ranks
There would likely be others as well (possibly the ability to cast 3rd level spells?), but these fall in line with the stuff you've mentioned specifically.
This way you avoid requiring levels in specific classes (which the guidelines say to stay away from), but there are still prereqs that insure certain classes are more likely candidates.
For example, Perform's not a class skill for many of the classes so if you're not a bard (or something like it) you're gonna have to be really high level to get 8 ranks. Even a Bard has to be 5th level to get those ranks.
A Cleric 01/Bard 05 could qualify under these prereqs (if you didn't add others, such as the ability to cast a minimum level spell). Other classes, such as Paladin, could still qualify but would have to be much higher level.
Also, it seem like rather than adding Animate Dead to the Bard list, instead you'd be making a list specific to the Dirge Singer PrC, and Animate Dead would be on
that list, right? 'Cause if you add it to the Bard list you're not making a PrC, you're modifying a base class.