Redirect DNS Requests

We are a small ISP with dialup and wireless accounts. We just inherited the
Dialup side of another ISP. Many of his users have statically assigned DNS
entries and some point at IP's that occasionally just don't answer. We are
on AT&T now.

We use Mikrotik Linux based routers. They have configurable firewalls that
allow things including Redirect and NAT.

Would it be possible to use NAT to redirect all UDP port 53 packets to
AT&T's DNS servers? Would this cause total chaos? We have tried it and it
seems to work for the users with the wrong static DNS servers in there
settings and there DNS lookups work reliably again. It has caused chaos on
one other occasion though. We have one user that is running a W2K box that
they use as there own DNS server and it goes directly to the root servers.
Some reason it quit.

So is redirecting DNS requests transparently feasible or will it cause more
trouble then its worth? I could imagine when a DNS client is expecting to
hear from a root or authorative DNS server and a recursive non-authorative
answers it might cause trouble. Would it only cause trouble in these cases
when trying to use the root servers? Would it help if I setup my own Linux
box running bind and redirect to that?

I am just looking for a way to fix and speed up DNS with the least effort
and problems. I also worry about changing backbone providers in the future
and we are not near big enough to own our own IP pools. So then the DNS
server IP's would change again!

Matt