There is a PR pending in ArduPilot that implements one of the fields in SET_POSITION_TARGET_GLOBAL_INT, however when the packet is very correctly checked for the appropriate frame type, there is no way for the autopilot to send a nack to the GCS (or sender), instead the best that can be done is to provide a status text message with the error as a warning.
There are two resolutions for this I can think of, one is a silent failure, and just rely on broadcasting the current position target to inform the GCS that it failed, however this is extremely poor as a failed command looks identical to packet loss, and there is no feedback to the GCS about why it was rejected. (Other feasible reasons include scenarios where the target position breaches a geofence)
The better solution would be to add a equivalent packet to the COMMAND_ACK packet for the SET_POSITION_TARGET and SET_ATTITUDE series of commands, and just continue with the existing series of MAV_RESULT enums.
Any thoughts from others? For reference the ArduPilot PR that prompted this was: https://github.com/ArduPilot/ardupilot/pull/4070