@pairjal
I think maybe you are over thinking this a little :)
Instead of using an EA to populate a Smart Groups that in turn triggers a policy, why not just combine all this into one policy that would do this-
Run a script that is a combination of your EA and your messaging script. High level, have the script do-
• Check UPTIME on the Mac
• If UPTIME is equal to or greater than x number of days (5?) then display message about being up too long to the user (with something like jamfHelper, cocoaDialog, etc)
• if UPTIME is less than x number of days, exit silently
Have the above run once per day on the every 15 trigger. This way you can be 100% certain that no-one would ever see a message about their Mac being up too long unless it really was. Having the script run on the Mac will gather accurate information and then take an action.
Does this make sense? I think you should forget about unnecessarily complicating this, personally.
If you wanted to make your script somewhat flexible, you could set up a parameter, $4, to pass a value to check for down to the script. In other words, if you decided later you wanted to check for 6 days or greater, just change the value for uptime days being passed to the script. No need to rewrite it and re-upload the script.
Lastly, to answer your question about recon for an extension attribute only, no, there is no way to do this. There are at least 2 Feature Requests asking for this though, and they're popular requests, but as of right now it doesn't look like it will make its way into the product. So you're not alone in wanting to do that.