Boy, this is one terrific thread!
haertig,
FWIW, I use Outpost Pro v2.5 (OP) for my firewall and I've also noticed the phanthom Avast.Setup.exe app and had some difficulty configuring it to work with Avast 4.x.
OP also has a MD5 checksum checking module (called component controll (c-c)) however, with OP you can turn off c-c either altogether or for a particular app.
Here's my rulesets. Maybe you can see something you can use.
ASHMAISV.EXE
TCP - outbound - Ports 25, 110, 995 (POPS if you need it.)
ASHSERV.EXE
TCP - outbound - Port 80
TCP - outbound - Port 80 - Loopback addy.
No c-c
AVAST.SETUP.EXE
TCP - outbound - Port 80
TCP - outbound - Port 80- Loopback addy.
No c-c
I had to add the loopback rules and turn off the c-c to finally get background and on-demand updates to get through.
You might want to check out Outpost. It's extremely configurable, works OOTB, and has that c-c per app option. It also fits in nicely with your (and my) preference for a slightly off-the-beaten-path firewall.

Hope this has been a help.
Quick update. I tried the email notification myself and it's ASHSERV.EXE that seems to need the SMTP rule.