Done it a few times with mixed results. My experience has been that the MT tends to run hot when accepting a pro level line signal, even with the L switch selected. If you get an output for which the sound engineer (or you) can and is willing to control the strength of the signal, you'll be ok.
If you get stuck with something else-say the outputs on the engineers dat recording deck, for which the strength of the output signal cannot be controlled, you may find that the MT can't handle the signal without some sort of attenuation device between the source and the MT.