I forget who it was but someone suggested using a pneumatic squeeze bulb like those used with an old camera to manually activate the Boost switch. A metal bracket could be mounted under the nut and washer holding down the adjacent pot. You can buy a shutter release bulb for $12.95 plus $3.50 s/h.
Squeeze bulb - Camera-wiki.org - The free camera encyclopedia
P.S. I find the stock boost circuit to be unusable as it bypasses the treble cap with a 0.047uF cap for a HUGE mid boost. I preferred the subtle boost of a 390pF cap but you could use a larger cap like .001uF or even .01uF. Because of the way the Classic 30/Delta Blues is constructed with 3 circuit boards folded in a U-shape it can be very tricky to work on.
One solution that requires no mods is to run a clean boost or graphic equalizer in the FX loop.