Well there you go! See below.

Brompton has been notoriously slow to innovate, even to adapt their design to use available off-the shelf systems. It was more than being conservative. I think it was because they were already selling everything they could produce. In recent years, under new leadership I think, and seeing more competition in the 349 trifold market, Brompton has been advancing technically. New frame material options, new wheel size options. But, for example, no Shimano 8 or 11 IGH, which requires wider dropout OLD, so a Scottish framebuilder offered his own custom rear triangle for such conversions, but at significant added cost for that and a new wheel. Though a long cage rear derailleur would not drag the ground according to Jipe above, I could still see Brompton being nervous if it was at all close to the ground; Brompton RD arm/cage is mostly forward, which also fully takes up the chain slack when folding. I'd be curious if the mount for that RD is same as for standard RDs. If so, and with a 135mm rear OLD, conversions should be easy for those desiring it.