You can lookup by PUID with the web service. For AcoustIDs there is no such possibility atm.
This is more relevant now, since we depredate PUIDs in http://blog.musicbrainz.org/?p=1832
The acoustid.org Web Service only allows lookup by actual fingerprint:
This should probably be implemented on the acoustid.org side, since that is where the data is. The corresponding ticket is:
This was originally requested in: