The AX25 protocol doesn't care what addresses he uses. If he's unable to
contact his address coordinator then there's no reason not to use RFC1918
addresses.
He _does_ care that these addresses are not in use on the Internet, because
if he wants his Windoze box to be able to contact (say) www.yahoo.com, he
must be sure that he is not already using www.yahoo.com's IP number! His
machine will NOT be visible from the Internet in any case, because his
network will not be in the Internet routing tables (unless he gets
additional IP numbers from his ISP). But using IP masquerading, even a
machine which is invisible from the Internet is able to access it, by
'sharing' the one valid IP number he has (the one which is given to his
Linux box by his ISP when he connects to the Internet)
Note that the job of the AMPR coordinator is not (or should not) just be to
give out two IP numbers at random. AX25 is a broadcast network and uses ARP,
and as such, numbers must be allocated so that all hosts on the same network
(i.e. frequency) must share the same IP number prefix. In this case, if the
frequency were not in use by any other AMPR users, the minimum the
coordinator could allocate would be a /30 prefix, which is actually a block
of 4 IP numbers (the first is reserved for the network number, and the last
is the broadcast address). More likely they would allocate a shorter prefix
(larger block of IP numbers) in case other ham operators came on line in the
same area.