Again, this is probably a stupid question, but do I understand correctly that I would not be removing the derailleur - that the new hub would work in conjunction with it just as the 3-gear hub does now?
You are welcome Jerry. Sorry I can't be more specific about my exact gear ratios but I'm pretty confident my lowest gear spins so fast on the flat that it can handle some mean hills! The only thing that remains in the mod is the plastic chain tensioner which is needed for the Brompton with some small modifications.
I note you were willing to switch to a Mezzo but that jefmcg hit the nail on the head that it won't really give you a hugely increased range than the Brompton.
If you want really low gearing what about a folder with the SRAM Dualdrive? 2 good folders with great gearing range are:
1) Dahon Speed Pro TT
This is on a closeout price of $959. The 2010 version shot up to $1359 but no significant difference. It's lowest gear is 28" but I'm sure Thor (Brakemeister on this forum) might be able to do something to lower the gears?
http://www.thorusa.com/dahon/2009/speedprott.htm
2) A custom made Bike Friday Tikit direct from Bike Friday. Folds quick and small. Another great folding bike. Again I'm sure they can get the gear ratio to the level you require.
You can either get a great resale on your Brommie or keep it depending. Modding the Brommie is not cheap although it was worth it for me! You go not have to use an internal gear hub mod either, some expert bike shops might be able to put a derailleur based system on your Brompton with the low gearing ratios that you need.