I think the -bios option will do it. For instance, -bios u54.u11
Failing that, there's always the glorious hack of copying the ROM image over top of the one it is currently using. It'll complain about the checksums not matching which won't matter. That might be necessary because I think it only allows you to choose a ROM from a fixed list of names.
Thanks but it does not appear to work as expected.
Peters-MacBook-Pro:mame0170-64bit pski$ ./mame64 trs80m2 -bios u54.u11
u54.u11: invalid bios
65-1991.z4 NOT FOUND (NO GOOD DUMP KNOWN) (tried in trs80m2 trs80m2kb)
FATALERROR: Required files are missing, the machine cannot be run.
EDIT: I tried renaming ROMs to u54.u11 and I think that hack seems to work. I guess it is hardcoded somewhere to load u54.u11?
Peters-MacBook-Pro:mame0170-64bit pski$ ./mame64 trs80m2
u54.u11 WRONG CHECKSUMS:
EXPECTED: CRC(823924b1) SHA1(aee0625bcbd8620b28ab705e15ad9bea804c8476)
FOUND: CRC(7a663049) SHA1(f308439ce266df717bfe79adcdad6024b4faa141)
65-1991.z4 NOT FOUND (NO GOOD DUMP KNOWN) (tried in trs80m2 trs80m2kb)
WARNING: the machine might not run correctly.