Gang: I think that this is a *very* tricky problem to solve correctly. It's already been noted that we're really talking about implementing a full-blown mysql proxy in iRules, and it gets worse (depending upon your implementation, of course) - imagine a statement like (taken from a random search on google for "mysql update based on select":
UPDATE products p set p.product_model=(select categories_name from categories_description where categories_name like CONCAT(p.product_mode,'%'));
Which I believe will need (or should) hit the same instance for both the update and the nested select that the update is based on. This may or may not be the best example, but IMO it's extremely easy to write a brittle rule that could break things later on, barring some very specific architectural designs and policies that will ensure you won't run into a use case like this.
-Matt