The idea of practitioners who become ākāśagamin is already in the Yogasūtra (4th-5th century? earlier?) which explains how to do it; pre-tantra. The term is probably older. YS 3, 42 kāyākāśayoḥ saṃbandhasaṃyamāl laghutūlasamāpatteś cākāśagamanam | Dan