Reading some PC discussions, it looks like RetroArch does have rumble currently, but it doesn’t seem to be hooked up on any Wii cores as far as I can tell. Apparently it’s enabled on other platforms when the controller type is set to “Joypad w/ Analog”, presumably since this is the “DualShock” equivalent mode, although I’m unsure of which cores currently include this feature.
In theory, the relevant cores for Wii are Gambatte, VBA-Next and Final Burn Alpha. There’s 22 rumble games for Game Boy Color and two for Game Boy Advance: Drill Dozer and Wario Ware: Twisted! (although the latter also requires motion controls). In addition, there’s a number of games which use the GameCube’s controller’s rumble when played on the Game Boy Player (GBP mode is apparently enabled by sending all four d-pad directions on start-up if anyone needs to test), although I’m not aware of any emulators that have supported this. On the FBA side, a number of arcade games, including major titles like OutRun, have force feedback. As far as I know, none of the other platforms currently emulated on Wii have force feedback.
I’m not sure whether RetroArch already supports rumble for any of these cores on other platforms, or which of them (if any) include rumble support in their standalone versions. I can’t seem to find anything about rumble in mainline Gambatte; as a source reference, VBA GX does rumble for both GBC and GBA, which I believe was implemented independently in the Wii port, beginning from r157. Final Burn did have a fork with rumble, and the Xbox’s FBA-XXX also included support for force feedback, but I don’t think the current Final Burn Alpha does. Source code in all of the above links.
The Wii has rumble for every standard control scheme, from GameCube pad to each of the Wiimote-based layouts–even the Classic Controller if you keep the Wiimote in your lap; commercial titles like Chocobo’s Dungeon took this angle, with rumble on the Wiimote during play even on Classic Controller. So basically the entire userbase–as well as the PS3, 360 and mobile (Android/iOS/etc.) users–have rumble-enabled devices available to them, so rumble is a broadly applicable feature.
Thanks for any interest.