Got the chance to mess around with this finally, got it working on the Wii and Dolphin.
I used the latest version
Here (RSDKv3-Decompilation/releases/tag/Wii-v3.0.1)
For now, you have to rename the RSDKv3 folder to 'SonicCD' for it to work with the forwarder, until that is potentially resolved.

v3.0.1 doesn't have the boring old terminal style aspect ratio selection screen as some issues seem to have been resolved that required it, it's in the release notes.
For clarification for anyone needing it, the packaging is as follows:
- root://apps/SonicCD/boot.dol, icon.png, meta.xlm
- root://SonicCD/Data.rsdk
Update:
Looks like the Mania port
Here (RSDKv5-Decompilation/releases/tag/v0.3.0) works too, this one needs the rsdk file within the app folder though. Maybe
@RunningSnakes can hook it up with another forwarder?
After some investigating and actually looking at time stamps etc though, I'm quite late to this party
I found your forwarder and the other goodies on
this thread