If you use any other time signatures like 6/8, 5/4, 12/8, etc the only thing you can do is to play the additional instruments/tracks yourself.
That isn't entirely true, I think, though I fully understand the sentiment and frustration.
The help text when one opens the time signature dialog says:
"Notes: For TimeSigs higher than 4/4, each bar is spread out over 2 or more bars (eg. 5/4 will be written as 3/4+2/4). For 12/8 or 6/8, use 4/4 with a triplet feel (eg. Irish). For 9/8 use a waltz .
Often the 6/8 or 12/8 are used expressly to accommodate triplet-based music, e.g. that "Irish" hint, or swing as used is, e.g., jazz.
The sad fact is that BiaB uses some hacks to get around it's basic 4/4 structure, which function OK but are, IMHO, a real pain to read. I really struggle trying to follow 5/4, for example' on the BiaB views.
Triplet/swing timing help get the rhythms OK. For notation and/or lead-sheets I use a notation program.