How wide of gear can you get with a single ring?
 

What is the widest single ring application you've ever seen?

Its all determined by the cassette. The widest cassette that will fir with your der will determine the widest gear range. I would say at least 11-27.

I presume that you mean gear range. The widest range MTB casettes I have seen are 11-34 as I recall. This would be a 309% range, almost exactly the same range as the Shimano 8 speed gear hubs. Actual low and hugh gear inches would be determined by the chainring size used. It would also require use of an MTB derailleur to handle shifts onto the larger casette sprockets.

11-34T cassette with SRAM DualDrive II three speed hub, Schlumpf High Speed Drive single ring crank.

That ought to do.

tcs

Rohloff IG is 525%.

I've seen a modified Capreo cassette that probably takes the cake for a non-internal gear system. The stock 9-26t was modified to 9-34t.