Hi Will,
Thanks for your email! I just checked, and indeed, that you can't access them. But, I don't think it would be terribly difficult (although likely a bit baroque) to implement the access protocol, and I think it would be a great addition. I think that it would require implementing a streamline data selector in yt/geometry/selection_routines.pyx. The streamline path object itself has a positions attribute, which gives the position at each point in the streamline. This can be accessed in the selection routine in an implementation of select_cell.
I'd be haappy to work through this with you, if you'd like -- you can join our slack channel (it should currently allow guests, or we can invite you) and we can talk through this if you'd like.
-Matt