In case you provide this interesting feature, would it be possible to allow it to support a custom command ?
The "Move" command uses RNFR / RNTO to move files and directories but this command may not allow to move files across volumes (ie. ioFTPD doesn't allow that). Some scripts provide this functionnality but they're called using another command like SITE MOVE <directory> <destination>.
IMHO that would be a "not needed" feature. This is a *very* site specific thing you mention, and supersedes the normal move commands, as described by some RFC. For a custom command it's better to just assign a short-key to do it for you.