Any input is still welcomed...

If I'm reading the documentation correctly, I think you can use the rightmost output from mc.groove~ to get what you need:

Out right outlet: Sync output. During the loop portion of the sample, this outlet outputs a signal that goes from 0. when the loop starts to 1. when the loop ends. Note: In order for buffer playback to begin (and consequently for sync output), one of the groove~ object's output channels must be connected to another signal object.

Unfortunately, that outputs the playback position relative to the actual loop length, not the position in the entire buffer.