I found a partial solution to this. I can use the AntiVir 7 on demand scanner without disabling the avast! providers, but I cannot get AntiVir to update with this method. I'm considering this a "work in progress":
Yes, I'll test...
It will work IF the user does NOT boot after the installation, COPY all files to another folder, UNINSTALL it right after.
There are a lot of Registry keys that 'make possible' Antivir to work and update. I cannot manage a way to do that without the Registry keys.
You can (although is very dangerous for common users) manually change the Registry to 'redirect' the keys to the new installation folder.
Anyway, as far I could see, there are two SYSTEM DRIVERS that are loaded EVEN WITH THE GUARD NOT INSTALLED.
They are: avgntflt.sys and avgio.sys.
I will NOT test what happens IF I DELETE this files or remove the Registry keys for them. Most probably, a BSOD (crash) will occurs with the absence of these two system drivers.
Besides all of these, tweaking and 'craking' the normal installation this way could be out of the 'legit' field... I don't know if it's permited or not...
The validity of the installation is 6 months (or at least untill August this year). Don't worth the test...
Avira has a very good detection rate but, right now, can't work as a second, backup, non-resident scanner :'(