You can probably last modified date by doing a search on the 'file' record type (i.e. treating it like a netsuite record) For the other approach, I believe you can overwrite the file by saving a new file to same folder with the same name.

Good idea with the search. Extra work but it would get around the object limitation.

For 2.0, it no longer overwrites the file if it has the same filename. It just silently fails, based on my experiene