With a 5-bolt compact crank, you can use a granny ring down to 20t. Thanks, shitmano, for changing the standard and screwing everybody that needs a super low gear.
Going further back, with a standard crank (74mm granny bolt circle) there used to be made a variety of adaptors to fit smaller rings than the 24t that's the normal lowest for that diameter. Avid made one that fit 20t and 22t compact grannies. White Industries made one that fit freewheel, as in thread-on freewheel, cogs down to I believe it was 16t. So did the guy who first raced the train from Durango to Silvertown in like '72 or so, starting the Iron Horse Classic. His bro was the engineer. Tom Mayer's his name, out of Albuquerque, I forget the original name of his little company, then he changed it to GIOS Team, God is our Savior, or God is on our Side, something like that to let the cycling world he found religion in a major way. Anyway, he had several adaptors, including one that added a fourth teeny tiny freewheel "chainring" behind a normal granny. On yer own making the front derailleur handle it, but it could be done if you were clever and ingenious enough.
Pretty sure all those adaptors died when compact cranks got popular.
And I think Bullseye cranks had, maybe still have, a configuration to take a ring down to 16t, if I recall.
Maybe the above adaptors are available on ebay, I never looked.
Another option is tracking down an old SunTour 14-38t, yes 38t, freewheel. I actually have one here, and took it apart -- it was a 5-sp. -- and fitted the larger cogs to a 6 sp. freewheel body. SunTour had a special derailleur for it, but I made mine work fine with a long cage shitmano deore. Not that I need it, just because I had it and it was a shop project.
So, to sum up, theoretically, using available technology from the recent past, you could have a 16t granny ring and a 38t large cog. Let's see, that, oh my, a 10.9 inch gear with a 26" wheel. Somebody tell Frank Berto (ask google if you never heard of him).