Not had it done, but when I got my M4 the dealer said Leica would do it: it's also the sort of job CRR Luton and Malcolm Taylor would do in the UK, and I guess similar businesses elsewhere.
While I'd have liked a 28mm frameline, I grew to appreciate the accuracy of the M4's framelines. The M6 finder - if we're talking 0.72 - doesn't have a wider angle of view, just more framelines, which have to fit into the same space.
I'd have thought it would be easier and cheaper to get someone to cut 28mm framelines into the existing 90mm mask, though they would necessarily be a bit close to the 35mm position.