an index to places already exists - George Broderick has produced a 7 vol (6 sheadings + extra) placenames of IoM which was partially derived from such sources - catch is that although he was sponsored by MNH the individual volumes are £100 each and are printed 'on demand' by a printing house in Germany, where he teaches.
I havnt used these to go back to a primary source but I assumes that being an academic he references where names were found.
Though GB will provide a name and a derivation thereof, a consolidated index to all sources quoting that name is I suspect beyond reach - though the improved search engine on my site can search for phrases (eg [red gap] ) the variation in spelling is very wide