We recently finished up a Bluesmaster 50 build and made a couple of mods. 

We removed the rock/jazz switch and added a slope pot to adjust the slope resistance in the tone stack.  The rock/jazz switch was kind of doing the same thing, but in a less effective way.  The mod provides a usable range of 33 to 108 kohm for the slope resistance.  You can get more distinctive Marshall vs. Fender voicings, and the volume doesn't change when you adjust the setting.  To do this, we removed the rock/jazz switch, replace the 47k r on the board with a 120k, installed a 1mA pot in series with the 47k r and installed that ass'y in parallel with the 120k r on the board.  Installed the pot in the switch hole (slight reaming required) and reconnected all the loose wires - which was pretty easy to figure out.  I can provide a layout if anybody wants it.

We added a pot to adjust the PAB volume boost level.  This was done by replacing the 68k r on the tail end of the mids control with a 22k r in series with a 50kb pot, providing a resistance of 22 to 72k.  We installed the pot through an existing hole which we slightly enlarged in the chassis located between V3 and the power tubes so it can be adjusted from the back of the amp.  Wired it back to the mids pot with shielded cable.  We plan to install a p/p pot in the mid-boost switch hole so the boost can be adjusted from the front and to keep the mid boost switching intact.

Yes - Your amp tech should be able to do this easily.  The last one I did, I used a push/pull pot for the slope control knob and used the switching function of the pot for the rock/jazz switch.  That way, the rock/jazz switch can be maintained while adding the adjustable slope function.