Yay!
The latest CVS snapshot has support for MMX conversion on BeOS
You’ll need the great NASM assembler, available from:
http://www.cryogen.com/Nasm/
To compile it with the free Metrowerks compiler you’ll need to
edit outform.h and put these lines at the top:
#define OF_ONLY
#define OF_WIN32
#define OF_DEFAULT of_win32
Then type ‘make -f Makefile.unx’ and copy the resulting binary, nasm, to /bin