Has any Buddhist ever talked about postponing Buddhahood forever, but not for all sentient beings? Has any Buddhist ever talked about postponing Buddhahood forever, but not for all sentient beings? 

I do not, I suppose, mean because the "bodhisattva" enjoys life. I am trying to ask about "compassion" for some group or even individual, if that is possible?

I don't mind if the answer ignores the alledged universality of Buddha-nature, but would prefer if it was from that perspective.
first karuna is not compassion. the pali word for compassion is anukampam.

second the buddha does not even claim he reached nibanna out of karuna, nor out of anukampam. The buddha claims that he taught out of anukampam though.