One source for base hardware:
Remember, if you start mounting things, you're only as secure as the weakest link. If you add a torquing, shock loaded (during a seaway, for example), or other dynamic challengest to underlying support situation, then you could do more than just break the seat support.
I would think, to satisfy your needs, that you could develop something that's removable with a backing plate -- assuming that the underlying bench seat will take it.
What boat is on? Do you have a picture is worth a thousand words... or a link to another sister ship's interior...