MPTM already supports this in a sense, insofar as that you can reference any external sample, including MP3 samples, which are then not stored in your module file itself.
However - this may be my opinion, but it is a very strong one and I am not going to move away from it - music production should always be done with lossless samples - using lossy samples degrades your song's quality, especially if you edit them and thus have to encode them multiple times. Lossy codecs like MP3 and OGG Vorbis / Opus work with psycho-acoustics which are calibrated exactly for the frequncy the sample was originally encoded at. If you pitch it up and down (which you do very often im modules), the psycho-acoustic model stops working and samples sound worse than necessary.
One of the few valid reasons for using compressed samples would be size-optimized games (e.g. for mobile platforms), and for that there's already MO3.

If you want compressed samples and compromise your sound quality - fine by me, use MPTM's external sample feature. Hard disk space is cheaper than ever, but if you really need to save space, I would suggest enabling <a href="http://forum.openmpt.org/index.php?topic=4961.0">lossless sample compression</a> instead.