The SRAM XXI system has a 10-42T 11 speed cassette with a single chainring.
It is designed for mountain bike use: for frames that can't use a front derailleur or for riders who have problem with dropping the chain from the chainrings of a double or triple crank. I haven't encountered either situation.
The downside is big gaps between gears.
Without using the SRAM components or modifying some other components, I don't think going larger than 34T will work, as you are limited by available rear derailleurs.
On the road, I think a rider who needs a very low gear (>34T) probably does not need a very high gear (a 11T), as he is not strong enough to use it, except on a downhill which is pointless, better to spin a high cadence or simply get in a tuck.