Hmmm... So it looks like there are the first 900MHz wifi modules, which claim 1 - 1.5km range outdoors, with data rates of megabits to tens of megabits in many circumstances. The module I have found is this one:
The only problem I see so far, is that there is no comment as to whether they can do ad-hoc mode, or only AP or station mode.
If they can do ad-hoc mode, then this is potentially very interesting for use in Serval Mesh Extenders.
More when I discover it... But if anyone else has experience working with these, I would love to hear from you.
UPDATE: It looks like it uses an NRC7292 chipset for the 802.11ah, for which source code is available:
They already have monitor mode implemented in the chip, it seems, so it looks like it *should* be possible to implement ad-hoc mode on it.