It seems that there's no way to access information about the currently playing track (ie: file location of the mp3), or to tell a deck to load a specific track (ie: take the file location for the track currently playing on Channel1, and tell Sampler1 to load the same track).
I'd like to build controller functionality that would allow sending a track to a sampler, syncing playposition, etc., then switching to the sampler as the sound source - to free up the deck to cue up another song as the last one is playing out. All of this can be accomplished with existing controls, seemingly, except for figuring out what file is playing on Channel1, and telling Sampler1 to load the same track. Syncing, etc. can all be done in script with current controls.
A hack for now might be to always engine.LoadSelectedTrack into a sampler via an engine.connectcontrol linked to LoadSelectedTrack (I'm assuming this would probably work, haven't tested), but it seems like it would be cleaner to only load the track into a sampler when it was needed.
This is sometimes called "instant doubles".