Haha, I thought the name Thangtong Gyalpo sounded familiar. I checked my cache of texts and found "the longevity practice - essence of immortality according to the heart essence of the great siddha". There is a line, " I invoke the heart samaya of Thangtong Gyalpo, gather the life-nectar" etc. Is this the practice you had in mind Cone?
Regardless, I would still need permission and instructions since I'm not familiar with it. Also a new copy since the text I have, looking it over, was a bad print and some of the mantras have ambiguous letters.