[Exim] Strange request.

I have exim set up on my local machine, and it's working great for local
delivery and such, but I'm wanting to expand things a bit.

Here's my situation.

I have a server on the net (SotN), that serves virtual domains as well. It
goes by alloyweb.com, befunk.com, brokenmachine.org, and a number of
others. That server is configured to deliver local mail to itself if it's
ranger@ (ranger@???, ranger@???, etc.). I'm using
fetchmail to pull the mail I receive there to my local machine.

My local machine (MLM) has an ADSL connection, and is on all of the time.
I have a couple of dynamic DNS accounts for it (defiance.darktech.org,
defiance.dyndns.org), as well as local DNS set up as router.localnet (for
reasons of letting my work laptop become a .localnet machine instead of a
work machine).

What I would like, is the following:

* local mail delivered locally, like normal.
* outgoing mail from my account (ranger) on MLM (and maybe eventually
others) to be sent as ranger@???, just as if I was sending mail
from, say, netscape.
* all other outgoing mail from MLM to be sent as
<user>@defiance.darktech.org

One of my big problems is the SotN is running a really messed-up,
cobbled-together Sendmail configuration that I couldn't change if I tried.
Also, I'm on a dynamic IP, so I can't really set up an MX record on
befunk.com to just allow relaying from MLM without really screwing with
scripts and MX records on the SotN.

If it's possible, it seems like the easiest way to do this is:

* rewrite outgoing smtp mail from ranger@MLM to be from ranger@???
* masquerade as befunk.com when sending mail to the outside world through
the SotN (I'm assuming this is how standalone mail programs like netscape
mail and eudora do it).

The second point seems to be a biggie, and I don't know if it's even
possible.

Do you have any insight on the best way to go about setting this up?

Thanks in advance for any help you can give me.