You want to increase healer/tank participation... you're gonna have to make DPS less desired (presumably by adding extra responsibility) for duties. And without going back and adjusting every single dungeon, you can't do that (before anyone says parser, that will also add extra pressure to the tank and healer, thus resulting in a zero sum gain in terms of DPS responsibility compared to healer/tank responsibility). And incentives won't do it either. All you're gonna get are tanks and healers that wanna be DPS, and will play them as such, rather than as tanks. Which makes the group suffer as a whole. Best thing to do if you want to see change, is to be part of the change yourself. And before you say "But you should take your own advice", I already healer main and tank sub.

Else... get friends, get an FC. For example for those DPS that need a tank buddy on Chaos, I don't mind chipping in and helping you out as a tank.