APM and Ping Identity - IdP initiated connection using query TargetResource=-url-

We are the IdP and setting up with a SP. When a protected resource is hit on the SP they want to trigger an IdP initiated session and provide a query parameter ?TargetResource=.

Described there at: https://ping.force.com/Support/PingIdentityArticle?id=kA340000000GsADCA0

I don't see any option to pick this type of information up from the initiating request and it always uses the target defined from the SP Metadata. Hoping someone may have already encountered this and am concluding it will require a custom iRule if it is, indeed, possible.