We have a 2820n on our system and we are currently using just the DSL connection for the internet.

We are in the process of setting up a wireless bridge to one of our other sites down the road, this bridge will be on the LAN side of the router. Is there a way of configuring the 2820n to use the 'internal' wireless bridge as an internet connection?

Yes; you set up a 'static route' on the Vigor, however, why do you want to do this rather than
your devices talk directly to your wireless link ?

Where we are we only have one option for internet access and that is through an ADSL connection. No fibre connections here.

We have another site just down the road from us, they have their own internet connection (again ADSL only, no fibre). We want to connect the two LANS together so that site 2 can use the servers on site 1 without a VPN etc. I was just curious if I could set the 2820 on site 1 to failover to the 2820 in site 2 for internet access via the wifi link that is on the LAN side of both routers.

I know it's an unconventional way, but it's also a cheap way to get redundancy on the internet connections as we cannot afford a leased line. and our ADSL only has 1.4mb upstream which isn't good enough for a VPN between the two sites.