also, if you REALLY want to pass all the files in the returned value, you might try replacing text in the files with identifiers such as $mircdir. you can use SendMessage with file mapping (look up SendMessage in mIRCs helpfile if you are unsure of what i mean) to evaluate identifiers suck as $mircdir. This would only apply if the files are within mIRCs dir though. also using the short filename as opposed to the long filename will aslo save you some length. even so you should still use custom signals rather than this method because there is no gaurantee that this list will be less than 900 chars.
