Is it possible to configure sendmail to use the hosts and resolv.conf files other then the ones located in /etc/?
In short, not that I know of. Most programs use standard system calls for any name resolution and leave the details to the OS.
Now, if you're concerned about people/programs changing file contents, take a look at the
One option might be setting up postfix and using transport maps to send email where you want it to go.
Bit much to post all the detail on that as it may not be what you want, but to give you an idea.
With this setup you can redirect domain names to wherever you want, regardless of dns on the box.