You might be able to accomplish this with the new KeyValuePattern object in Mathematica 10.4. I've tried, though, and can't seem to figure out what function should then be used to actually apply the pattern to select matching key->value pairs from an association. I crafted a test association that is similar to yours, with keys that are lists, and was able to get MatchQ working with a KeyValuePattern such as:
MatchQ[testAssoc, KeyValuePattern[{{20, _, _} -> _}]]
That returned True, as it should, but I couldn't figure out how to actually return an association of key->value pairs that match the pattern.
