Is it possible to restrict a device from registering during SRST mode? and if so, how would it be done?
Yes, this is possible. Create two device pools on Callmanager for the phones on the remote location. Keep the phones that you would like to register in SRST mode in the DP that has the SRST reference. And for the phones that you would not want to register to SRST gateway during WAN failure should be in the other DP that does not have an SRST reference.
Everything else in both the DPs can be set same, just one of the DP should not have SRST reference.