[Taqwus]

The Ankh turns you into Soulless, Twiceborn makes you a wight mage. Both preserve magic, location, items, etc (well, Nataraja loses two arms I guess, but other than that...). In both cases, transformation is one-way.
You not only have to die, but lose the battle as well or at least make sure that none of your friendlies have that slot open.
Empowering Nature and using Transformation should work because many of the new forms can't use armor, but you can't transform back to regain that armor slot... although you might be able to keep transforming until you turn into something with an armor slot again.