Does MYSQL join before LIMIT?
The only way around it is to refactor the query so as to alter the order of the EXPLAIN plan.
EXAMPLE : On May 16, 2011, I once answered a very rough question on StackOverflow
Fetching a Single Row from Join Table

Question : I had to join tables together and return only the 40 most recent results.
My Answer : I proved that you can do a JOIN on tables with large rows and get results fast.
How I did it : I had applied principles of refactoring queries I learned from a Youtube Video.

Basically, you have to construct the query so that JOIN clauses are done last. Consequently, this implies that you must get the LIMIT executed beforehand.