I don't own a Smash, but it looks like the two sets of outputs are wired in parallel from one set of outs. If this is the case, and there is enough real estate to the left of the outs, behind the transformer, the output wiring could be extended (ideally during the initial build) to reach a rotary switch. The switch would be wired like the input switch. A 3/8" diameter hole would be the only required modification to the panel.
Do you really need the Smash for either/both the SEX and/or the SM? Both are integrated.