Trevor & Amol - Thanks for the thoughts & responses.
Trevor - We also opened a message asking for a more efficient way to manage this process, but were essentially told no such solution exists beyond legacy file system sync. Unfortunately for us creating the non-SAP roles directly in GRC as template role type or even business role type was not possible as all of our requests come into GRC via webservice submission via an IDM tool (rather than being submitted within GRC through its Access Request Form) - template role type & business role type are not supported via IDM/webservice provisioning.
We have ~60 non-SAP systems to provision for so the prospect of managing 60 different legacy system files & structures just to sync roles that we've already loaded into BRM seemed a bit burdensome. After brainstorming various options we came up with for our scenario was an 'easier' workaround:
We chose to actually create our non-SAP roles in an unused ABAP system as PFCG roles (just role name & description were maintained and loaded via eCATT). We then changed our ~60 non-SAP connectors from SM59 Logical Connectors to RFC Connectors. We pointed each of these non-SAP RFC connectors to the same back-end ABAP system were we had created our non-SAP roles. We were then just able to leverage the standard ABAP system RFC repository sync programs to bring these roles into the GRC repository so that the GRACRLCONN table thinks they "exist" and are available to be requested.