We ship out the 5.1 dll and use the FF_SCAN on the fly too. Are you using:

*udir points to the directory the table is located in
*ufile is the tablename
=FF_SCAN(udir+ufile+'.DBF',-1)		&& -1 = fix all errors

We have not noticed any problems using it to repair memo files. We also have a separate app that steps through all tables in a directory, does a complete scan, and has been fixing the memo files successfully. IF a file cannot be fixed, we archive it, create a new table from scratch, and bring in all of the records from the original table. That is only run when a user exhibits numerous file corruptions though to ensure all tables are are in good condition.