[Hellstrom]

Blackguard Command Undead may not be the best thing ever, but that is straight from source. Although I guess I could make it so it takes character level instead of class level. Source doesn't specifically say it's one or the other...

Yeah, like I said... a minor point. Especially b/c if you made it character level, then you'd have to worry about double counting cleric levels in the Cleric/BG multiclass situation. e.g. You want a C8/Bg4 to command as a level 10 cleric, not a level 18 cleric. (Because 8 cleric levels + 12 character levels as BG = 18)

Whatever I would create, so long as it uses the EffectPoison() function, I will not be able to modify the DC on-the-fly. The alternative would be to implement a custom poison system that doesn't even use that function at all, but instead makes DC checks in another way, then applied the poison separately. Although that would still mean using the EffectPoison() function to apply the poison, so it's still not ideal. All-in-all, I just don't think a 'floating' save DC based on the PC who applied the poison is going to work due to game mechanics.

Right. I was suggesting something that might let you work around not having floating DCs. I'll explain in more detail in case you find it useful.

Basically you could have two categories of poisons, some that can be used by anyone, and some (with the higher DCs) that can only be used by those with the Use Poison feat. To reduce confusion about which are which, you call the 2nd category "Toxins" rather than poisons. Here's a quick example:
Giant Wasp Poison, DC 18, can be used by anyone (with an Apply check for those without Use Poison feat)
Giant Wasp Toxin, DC 21, can only be used by those with the Use Poison feat.

Does that make sense?