FreeSwan + backup line

I have a simple network:


------- ---------
192.168.0.1--| GW 1 |--INTERNET--| GW 2 |--192.168.1.1
-------- --------
| |
-------- --------
| Isdn 1 | | Isdn 2 |
-------- --------
| |
--------------------------

Gw1 and GW2 are two FreeSwan gateways to implement a tunnel from
192.168.0.0/24 and 192.168.1.0/24 nets. Isdn 1 and Isdn 2 are tho little
pocket isdn routers for dial on demand (this router don't supports RIP/OSPF
or other dynamic routing protocols). I whish to use the isdn line as backup
line, so the dial must occours only when the tunnel go down.
A way may be to insert a static route for isdn in both sides with an hight
metric and use RIP (or another dyn routing
protocol) inside GW1 and GW2 for remove or make alive a route in the routing
tables. Is this possible ?
Can i do that in another way ?
I hope your (my?) networking capability a little better of my English ! :-)

Regards