Howdy,
I've a rule which is blocking or permitting access to certain URI's by means of a starts_with match. I can match "/testuri" against the request uri, e.g. /testuri/example and then ...
I don't think you're able to change the execution order for anything really (other than between iRules as you mention). I think your only route here is to do it all in an iRule I'm afraid.