A new patch release 2.0.3 of the MD5 implementation for Common Lisp has been released, which includes support for md5sum-string for as yet unsupported Lisp implementations through flexi-streams. This enhancement was supplied by Daniel Kochmanski. The new release is available from its PMSF page, and its GitHub home.
Note that MD5 has been cryptographically broken, and should therefore not be used in situations where a cryptographically secure hash function is needed (you might want to consider using our implementation of SHA-3 in those situations instead). However due to its wide historical use and its remaining suitability in non-cryptographically secure settings, this implementation is still of some interest.