Anthony,
you probably created your target from some of the existing targets. Most
of the targets have the configuration parameter 'asmOpts'. You could add
following to your target in config.bld ONLY when you build BIOS 6.
I haven't really tested if the make function calls I used are just right,
so you might need to make some edits but you can tell where I am going
with it.
This is a hack that relies on knowing what's inside our generated
makefiles, so if you decide to try it, you probably want to use it only
until the BIOS 6 build system adds some options to let you specify per
file command line options.