I know you're probably saying 'Why not just buy a tone wicker?', well a) It means spending more money on something and b) I thought it might be cheaper to mod a toggle switch with the same function as the Tone Wicker onto the standard Big Muff Pi.
Is this at all possible?
Yes, but it might be a bit fiddly. From what I can gather, it is simply lifting the three B-C caps on Q1-3. You could remove them from the board and mount them on the switch, or lift one leg on each and run wires from the lifted legs + holes from whence they came to the switch.
GGG has the schematic (http://www.generalguitargadgets.com/projects/17-distortion/108-big-muff-pi) showing how to do the tone bypass for transistor muffs too.
nocentelli is correct regarding the tone wicker mod. To lift all three caps you'll need either a 3PSP or 3PDP switch. The latter of the two might be easier to find.
The tone bypass will give you a volume boost, BUT will also give you a very noticeable increase in noise as well. Here's another option for modding the tone control: http://guitarkitbuilder.com/content/big-muff-pi-mod-more-mid-range-tone (http://guitarkitbuilder.com/content/big-muff-pi-mod-more-mid-range-tone)
Using a switch, you can make the tone control selectable between stock, scooped mids, and flat mids. Download the tone stack calculator and you can tweak the tone control as you like. http://www.duncanamps.com/tsc/ (http://www.duncanamps.com/tsc/)
The actual EH Tonewicker BMP uses a single pole toggle switch, and a 4066 (switch IC) to switch the 3 caps out - using something like this, or optocouplers would be more difficult, but less likely to cause popping from one lead of the capacitors being left floating.
Thanks guys, I'll have a look at what you've all posted.