Creating iRule for web proxies with site preference

In the 10 mins since I wrote the question, I started to think I've been going about this the wrong way..... Based on the above, the proxy setting in IE on the clients will have to be the IP of the Virtual Server on the LTMs at one of the sites. This could result in traffic going to the HQ site before then going over the WAN to the DR site to use the internet link there.

I feel I should add here the logic behind this - we have a high speed internet connection at the HQ site and at the DR site. Currently the clients all point to the HQ site proxy and therefore use the HQ internet connection. Very little uses the DR connection. We are about to start migration to Office 365, hybrid Exchange and Skype for Business, so want to make best use of both links for this increased traffic.

We have a GTM at each site and although configured as authoritative for a stub DNS zone, they aren't actually used for anything. I'm thinking that the client proxies should actually put to the GTM (i.e. proxy.stubzone.domain.local) to cover LTM failure at either site. Therefore, the iRule for the site preference should actually be running on the GTM. (I hope that makes sense.)

So, given that, would this iRule, or a variation of it, work on a GTM and still meet the requirements?

Many thanks, Andrew