Is it possible to make a cable that work as a permanent "ground to tip" switch simply by plugging it in?
It's for activating a mode on my Mesa 2:90 Poweramp.
My band:

Sure, you just have to connect the two contacts. You can use a plug with the two contacts soldered together and no cable, or cut a broken cable and connect the two elements. If you want something with a smaller profile, you can use a 1/8 to 1/4" headphone adapter with some solder dripped into it.