IPS exception based on Proxied Source IP?

[LPsUsername]

I'm OK with adding IPS exceptions based on src, dst, port or protection, but is it possible to add one based on just the proxied source IP?

The scenario is we have traffic coming from an externally hosted vulnerability scanning service taking the following path:

External source --> FW --> reverse proxy --> FW --> web server

I can add an exception based on the "real" source the first time through the firewall no problem, but on the second time through the source is now the reverse proxy. I don't want to add and exception for everything from the reverse proxy and I don't want to block these scans either.

Any way to allow the good scan through but keep blocking the bad stuff?