It already works with freearc. However it looks like you have to use solid=0, therefore you cannot use --jobs option I guess.
Code:
[External compressor:balrogg]
header = 0
solid = 0
packcmd = balrogg64.exe -1 e $$arcdatafile$$.ogg $$arcdatafile$$.blr
unpackcmd = balrogg64.exe d $$arcdatafile$$.blr $$arcdatafile$$.ogg
datafile = $$arcdatafile$$.ogg
packedfile = $$arcdatafile$$.blr