Depending on the platforms the other datasource(s) run on, and on how up to date these reports need to be, my instinct would be to replicate all the data into some sort of warehouse (on MySQL if appropriate) and let the users carry on using the syntax they know.
This would seem to be far simpler than the parser you're planning (which seems kind of like re-inventing the ANSI SQL wheel), although I appreciate it may not be appropriate for your environment.
Anyway, good luck with it.