Actually you can tell the operating system to always use neolemmix to open files with a certain extension.
I do not know if neolemmix can search the correct level from the replay. I added it in Lemmix and it works quite good.
NeoLemmix does have ways of matching replays to levels, but it's designed for verification purposes, not for search purposes - basically, to test "does this replay match the level it was loaded on?".
Nonetheless, it
can - albeit a bit slowly - be used for matching, as the Mass Replay Check relies on this. So it's not out of the question. There's extra metadata in the replays that could further help with the matching, though isn't 100% reliable (in particular, the metadata will be wrong - except for the level title and level ID number - if the replay was recorded in testplay mode rather than after the level was added to a pack).
You can associate NXRP files with NeoLemmix.exe, but it won't do anything. Only NXLV (level) files will.