I think the easiest way would be to add a method in the file "runtime/parrot/library/Digest/MD5.pir" that gives the md5 checksum back as a string. I can look at the source code and try to do this for you if you want and add a method may be called "_md5_sum_get".
I will try to do that, if you don't mind. Maybe I can learn something more about parrot and rakudo along the way. Thanks, -- Cosimo