I would like to know the TW's answer to this thread's question, too.
I played with M&B Warband using GFN, the sligth mouse delay is hardly recognizable so the SP campaign was perfectly playable for me.
I know two games were added in EA stage to GFN (My Time at Portia, Dungeon Defenders Awakened [still in EA]) so it is possible. Some M&B fans already asking about M&B Bannerlord in the GFN's forums.
I really hope that TW and NV are going to make a deal on this.They surely already know each other, the NVIDIA Shield was promoted with the android port of M&B at that time.