"...that could preserve all the "BIAB" stuff".

This is the key reason for the suggestion to save a .SEQ.

While RB can indeed open a .MGU file, it does tend to loose some of the details. For example, try changing tempo in the middle of a chorus in BIAB at the B section. Works good in BIAB but open the same .MGU file in RB and the real drumer sits this one out.