This is more like a possible bug report, rather than a suggestion.
The SimConnect_SetInputGroupPriority API call is currently reported as "No error", instead of "Working" and in fact, after a couple of tests, it seems it doesn't do anything, no errors but also no effect.
The goal is to be able to temporarily map a key or a joy button, and prevent it from reaching the sim, if the priority is set to SIMCONNECT_GROUP_PRIORITY_HIGHEST_MASKABLE and the client events has been added to the input group with the bMaskable flag enabled.