I don't have Apache handy to test, but some combination of these rules should do what you want:
RewriteEngine on
RewriteMap lower int:tolower
RewriteCond ${lower:%{REQUEST_URI}} -U
RewriteRule [A-Z] ${lower:%{REQUEST_URI}} [R=302,L]
- A lowercase map to convert /SoMeThinG to /something
- A condition to see if the lowercase of the REQUEST_URI exists (-U is internal apache query)
- The rule to actually do the rewrite
I don't know if the RewriteMap can be applied in a condition, or if it only applies to a rule. These are based on experts exchange accepted answer and a small orange forum discussion.
Your "ideal" solution is probably not possible unless you can enumerate every valid page on your site. If you only have a few valid pages, a combination of RewriteMap and a text map will do exactly what you need. If there are hundreds / thousands of pages you may need to write a script and use the prg
directive.
If you can't identify every valid page, you would need to try every variant in case. Consider your URL as a binary string, with 0 for lowercase letter and 1 for uppercase. Just from your simple example you'd have to test 2^17 variations, 128k pages.