Nico de Haer wrote:
> Yes, both ADSL routers have support for IPSec passthrough. I also have full
> control about both the routers.

Then I would just give it a try and configure a site-to-site IPSec with 
pass-through enabled on the ADSL routers.
For testing purposes you may use single IP address as local subnet on 
both sides to see if it works.

It only depends on the pass-through feature and, if this works, on the 
official IP address that you say sometimes will change. You then will 
have to manually correct the IPSec configuration!

-Bjoern