You got it. It's already there although not documented...
To customize the text, open the files <avast4>\data\avast4.ini and add an entry called VirusMessage into the [UserInterface] section.
This entry is the actual message body that will be placed to the SMTP alert messages. The following placeholders can be used in the definition:
/n breaks line
%1 full path name of infected file
%2 name of virus detected
%3 name of the avast task that detected the infection
%4 version of virus database (VPS file)
%5 name of the computer where the infection was detected
So for example, you can have the following in your avast4.ini
[UserInterface]
VirusMessage=Something wrong dude!\n\navast (VPS %4) found virus %2 in file %1 (using task %3)\n\nDo something dude!!!
Hope this helps,
Vlk