My other option, of course, is to make the discipline switch by sheer force of will, viz., studying on my own, attending conferences, and trying to publish in the field until I can gain a modicum of credibility. An awful lot of philosophers started out as theologians, classicists, and mathematicians anyway.
The reason going back for another PhD is appealing is it would let meto devote myself to this full time, rather than having to work a day job to make rent and pay my loans. It would also be useful for me to make personal contacts; I failed to do this effectively while earning my first degree, and it is costing me.