I'd be interested to see the results, but I'm pretty sure you can't use ! in the middle of a pattern. In vanilla Regular Expressions it's only used in lookaheads/ lookbehinds; its broader "not this" meaning is specific to mod_rewrite. Also some other Apache mods, but never mind those.
Picking apart the pattern as long as I'm here ;)
begins with literal text Libraries/Emulation/NES/
(([^/]+/)*) = there may or may not be additional directories here. Since you're ultimately looking for something with an extension, you can shave a couple of nanoseconds by expressing the inner bit as [^/.]+/
[^/.]+ = a final bit of plain text excluding directory slashes and periods
\. = literal period
So far it looks really good. It's just that mid-pattern ! that I'm leery about.
:: detour here ::
OK, I went and tested it in my art studio's site which never gets any traffic. I'd actually expected a 500-class error, but all that happened was that the rule didn't execute. I deleted the ! and it worked as intended: that is, it redirected a jpg request to the nonexistent page I'd specified.
But there's got to be a finite number of possible extensions on your "real" files, especially if you've already constrained the search to a particular sub-sub-directory. For example, you don't want to redirect css files do you? (Was going to say robots.txt, but of course it doesn't live in a triple nest of folders ;) )
So the rule will work nicely if you replace the !(jpe?g) et cetera with the desired extensions, say (jsp|php|html?) et cetera.