I'm sure this will not change your desire to have such message, but consider this:
Those messages serve mostly as ads for AVs, that's what they are, and, security-wise, they are just cosmetic, any reassurance they might give to anyone is false because they can be easily faked, as such people should really stop seeing any sort of security assurance in those, and they shouldn't trust an email just because it says it was scanned with AV X.
Avast does something which might be more useful, Avast! marks the headers (something that is normally hidden in email apps) with something similar to this:
X-Antivirus: avast! (VPS 14061801)
X-Antivirus-Status: Clean
This helps with filtering of email made server-side.
But still, this header can be easily be faked also.
Anecdotally, you can just add "This email was scanned using an up-to-date Antivirus"/ "This email was scanned by avast! for mac and found no malware" or similar to your signature.
Really, having the AV writing that message wields no magic power, this 'seal of approval' holds no credibility, nor an email that doesn't have it is less credible for that reason, if the email is found to have a virus then it's blocked, it's up to the receiver to trust their own software they installed for scanning email, not other people's software.
Also, on that subject, most email servers do virus scanning silently (Gmail does it, hotmail does it...), viruses are very rarely transmitted as attachments nowadays, normally it's email messages with links to the web to the viruses (posing as being something legit). Hell, you might even forward that message and have Avats!/AVG/... mark it as scanned (because there is no virus on the message) and you are sending your recipients a doorway to a virus. You can see how those messages serve no purpose except instil a false sense of security which is more harmful.