Story so fr:
1) PPP on-demand with static IP works.
2) PPP on-demand with dynamic IP says "Host is down" on any IP request
The problem is that tun driver check its READY state by *first* ifconfig address.
i.e.:
set ifaddr <addr> <addr2>
works (static IP) and
set ifaddr 0 <addr2>
not works (dynamic IP) because first address is equal 0.
Since tun is always POINTOPOINT interface, dst address is more meaningfull.
I change checking to second (dst) address in READY test.
PPP on-demand finally works.