HorvatM has released a new, superior assembler for the V810 processor used in the Virtual Boy, called simply “Matej’s V810 assembler” or “MV810ASM” in short.
It has a working expression evaluator, local labels/identifiers, can automatically assign addresses to global variables, supports VB-specific instructions, recognizes both the “$” and “r” way of referring to registers, automatically aligns everything, has clear error messages, and you can just use it in any way you want. It also has a ROM-hacking mode with which you can apply a hack to a ROM in one step, no hex editing required.
You can get it for multiple platforms on Matej’s Website. For more info and discussions check out the respective forum thread.