Not being very familiar with rogaining I have never heard of this before, but it sounds like something very civilized. Is it common?
In eastern Australia it is. We don't offer that option on the west coast. It's either 24hr or nothing, though you don't actually have to go out for 24hrs.
Roving events crept into SA a couple of years ago but this was the first one I had done. At least it was as part of a proper 24-hour. Later in the year there will be a roving event with 15 of 24hrs or 8 of 20hrs; not sure how I feel about that!
15 in 24 is THE best! Still have to do some night but not all - good decent break and can push hard either side. (And as usually I'm in HH I love seeing people coming and going throuhgout the whole period.). I admire the 24 hour people immensely but 15/24 is a really different strategy. (and different strategy for HH)