a few days ago i adapted a hosa dual for a studio 22 plus. the plus uses an led with a resistor single footswitch for the gain on setting, and a separate standard single button footswitch for the eq on/off. i found the schematic for the gain footswitch on the amp schematic and wired it in the hosa left button, which will be black on the stereo to mono Y adapter.
so i plug a Stereo cable into the footswitch, couple it to a Y stereo to L R mono adapter cable, and into the respective jacks on the amp, works perfectly.
it should work perfectly for you too, but you'll need to check the specific footswitch wiring for your amp from the amp schematic if the switch schematic is not available separately, or contact mesa boogie support, they are very helpful. i recently worked with a new guy Brent there, he was awesome at finding info for me on a different matter.
Note, the hosa dual switches share a common ground per the trs cable. you may have to try a few configs to get that right.
the hosa is not that hard to work with, just modify it for what you'll need the switch to do. in my case i saved a hundred, and have a better solution. they make this stuff based on big picture production and multiple line considerations. virtually every guitar gear manufacturer has price point to production methods or considerations that give them an edge in their respective markets.
