Shame on me not to think of that aproach earlier especially as I know
vde.sourceforge.net already.
This might be aproach where no codechanges at all might be needed.
VDE is the equivalent to an ethernet-switch realized as a userspace
application. There are several plugins available. Amongst these
there are a tap-plugin and a slirp plugin. Haven't checked enough
to see if there is a pcap plugin as well. If not it is probably not
complicated to write. But maybe we wouldn't even need it.
Haven't checked about bridge & tap - support in the official
freemantle-kernel. Does anybody know if these features are there?
If yes the solution would be to setup a vde-switch with a tap and
a slirp - end and to bridge the tap with the wlan0 or pan0 device.

As from what we have seen (kernel module piping data to user
space) regarding Joikuspot on N900 this guy obviously hasn't
found a way to handle the NAT in the kernel either (without building
a new kernel).
So I think my aproach will perform at least as well as this one
with the two advantages that it is free and that the userspace-nat-code
used is really well tested in the virtualization world (at least qemu, kvm
and virtualbox) and as it is open-source any application-support
missing can be added instead of being bound to the protocols supported
by a closed source product. Btw. does anybody know which protocols
are supported by the Joikuspot-NAT-code - probably no difference to
what works on the symbian version. I'm thinking of things like active FTP,
IPSec-Tunnels, telco-protocols... where e.g. the linux-kernel has
Nat-Helpers for.