I know this is an old question, and this is a bit hackish of an answer, but you could simply make a Group Policy Preferences Registry item to disable the screensaver when the session name contains RDP. You could set up the registry item to set the timeout to something ridiculous like 9999 minutes:
Then set up the targeting to only apply when the session name matches an RDP session. There's no wildcard, but the number is incremented at each RDP logon, resetting at reboots, so just put in a bunch.
Not exactly an elegant solution, but it would get the job done. Make sure you create this as a user policy, and check the "run in current user context" box.