I have been thinkking about this recently.
I put a TS 2-3 on mine recently to run an EIS relay in the Leslie. In the EIS leaflet it said an added foot switch would only give you stop, or what ever speed the TS 2 was at. So I thought the only thing you could do is have a double pole switch intercepting the wires from the TS 2 to the hookup kit so that you can use EITHER a foot switch or TS 2. I think Bob at EIS is developing a controller but there seems to be a delay due to poor health.
If you have a standard Leslie relay ignore my bit about EIS but I still think the only way is to have a switch to toggle between footswitch and TS2