Its actually easier than I thought, here's a workaround:
1. Open Simshaker for Aviators and Simshaker Sound Module
2. Open DCS
3. Use notepad to open the file user/savedGames/DCS(or openBeta)/Scripts/SimShaker.lua
4. Change the line
elseif obj.Name == "MiG-29A" or obj.Name == "MiG-29G" or obj.Name == "MiG-29S" or obj.Name == "Su-25" or obj.Name == "Su-25T" or obj.Name == "Su-27" or obj.Name == "Su-33" or obj.Name == "F-15C" or obj.Name == "A-10A" or obj.Name == "A-4E-C" or obj.Name == "J-11A" then
to
elseif obj.Name == "MiG-29A" or obj.Name == "VSN_F104G" or obj.Name == "MiG-29G" or obj.Name == "MiG-29S" or obj.Name == "Su-25" or obj.Name == "Su-25T" or obj.Name == "Su-27" or obj.Name == "Su-33" or obj.Name == "F-15C" or obj.Name == "A-10A" or obj.Name == "A-4E-C" or obj.Name == "J-11A" then
You have to edit it after opening DCS, because the program overwrites SimShaker.lua every time you open DCS. For the same reason you have to do this edit every time you have opened DCS.
5. Do a flight in the F-15C, so that Simshaker registers the current aircraft as an F-15C.
6. Then fly the VSN F-104G, with a little luck Simshaker thinks its the F-15C, and gives you feedback just like in the F-15C.
You have to follow the above sequence, otherwise it doesn't work.
Both Simshaker for Aviators and the demo version of Simshaker Sound Module are free, so you can try this without paying anything, if you have a Buttkicker or a similar device.
Here's a video I made to show that it actually works, the graph on the right side shows that stall shaking is activated when I overpull the plane and the angle of attack is too high: https://youtu.be/_YNyK_XYA7M?t=35
As far as I can see, it works like the stick shaker that the real F-104 had, to warn the pilot before the plane stalls.
Hopefully the developers of Simshaker can make a more permanent solution, it looks like it shouldn't be too difficult.