Thanks everyone for the suggestions!
Wysicom PTT ring.
https://wisycomusa.com/product/ptt301/
What I was looking for, but Wisycom says that will only work with their transmitters which I don't have any of. When I called them this morning they said the threads are unique and it won't work with other brands.
+1; the SKM 300 (and only the 300) has this feature in the current G4 version as well.
This seems perfect. I have a 2000 series receiver sitting on the shelf which I can use with an SKM 300. I'll only need to purchase the transmitter. The manuals also confirm this option.