That's basically what I did with SWMBO's Brompton. Put in a 5 speed wheel and the shifter underneath. But I had to modify it.
Pix of shifter?
I have a out of stock plastic Brompton 5 speed shifter that I have been thinking of using but it is plastic.. it is a HSJ968 SLW5N Thumb Shifter.
I have been looking at the option of using a bar end shifter too. Anybody did that on a M bar B?