Dear Dan, Paul and others who replied,
I tend to accept the Yogācārabhūmi Viniścayasaṃgrahanī at the moment as the most probable solution. The Tibetan commentary I work with was composed in 1633, but it is based on texts that were already available since the late 13th century. I will have to dig into the older material to see if a gTan la dbab pa is mentioned, and if it is, how it is treated.
Thanks for everyone's consideration of the matter!