I have never encountered any ultra, where it is stipulated that entrants must walk part of the event. The only question is whether you can walk round within the cut off times.
Elite power walkers routinely take on ultra distance events. For many years it was assumed that it was impossible to walk round the Lakeland Trail 100 within the cut off times, but the British Champion recently proved otherwise. In the Thames Path 100 yesterday, one of my friends injured her knee 15 miles into the race, and power walked round the course in 25 hours.
I would submit that even within ultras that are conventionally only undertaken by runners, there are plenty of possibilities for you.