Is it possible to get the matched regex from within the apache LocationMatch directive?

LocationMatch doesn't support backreferencing, you can't do that in versions before 2.4.8.

According to the Apache documentation this is now supported...

From 2.4.8 onwards, named groups and backreferences are captured and written to the environment with the corresponding name prefixed with "MATCH_" and in upper case. This allows elements of URLs to be referenced from within expressions and modules like mod_rewrite. In order to prevent confusion, numbered (unnamed) backreferences are ignored. Use named groups instead.

<LocationMatch "^/combined/(?<sitename>[^/]+)">
    require ldap-group cn=%{env:MATCH_SITENAME},ou=combined,o=Example
</LocationMatch>