SSH and tun devices: which one?

Is there some way to determine the id of a tun device allocated by ssh on
the server side when the -w option is used? I'd hoped for a nice easy
environment variable, but no such luck.

I do note that an "lsof -p $PPID | fgrep /dev/net/tun" identifies the
open file descriptor. But I've not yet found a way to map this to the
tun device.

I know that the device can be *specified* with the -w option, but I'm
hoping to avoid having to "preallocate" devices to users.

Any suggestions?

Thanks...Andrew