Apologies on the confusion, IRuleYou. They are one and the same - both are relevant to my thinking (a) evaluate input from a custom aspx (text that comes back) then (b) make a decision between X and Y.
Perhaps I should have created two separate posts (to avoid the confusion) but from all the posts I have read 'the big picture' is usually relevant and often folks (like you!) have a far better solution than what us rookies are thinking!
For clarity, I certainly do not want to 'manipulate the monitor' but just 'read the result' - or, I was thinking of 'calling the monitor' from the irule (to get the result). The word 'manipulate' is a bit invasive and I wanted to be sure I was communicating this properly.
Those links (Thanks, Bhattman!) really showcase the flexibility of the LTM (to host the down page! nice!) and I appreciate the guidance.
I still can't figure out the 'between X and Y' portion of this, but have plenty to work with now. Another leading thought for this project is to build the time values in the aspx itself (in other words, just make the f5 monitor fail between X and Y by sending back the wrong text to the LTM, we'd write the aspx to monitor a backend database where we could set the X and Y flags) - so rather than build the logic in the LTM via irule, just 'down the nodes' by making the monitor fail at our discretion.
We're still kicking the idea around...