I can’t speak for Alwil, as I don’t know what packers are supported or what might be supported in the near future, but avast has one of the best package of supported packers.
I don’t know how easy or difficult it is adding additional packer support.
One of the other problems is the fact that the Eicar test virus has to be in a certain format and some have made additions to that format that don’t conform to the standard format, in those cases AV’s that are looking for the correct format won’t find it regardless of the packer used, supported or otherwise. So it is a little more complex than simply adding packer support.
You could send the undetected eicar tests files to avast!, if you can zip and password protect (‘virus’, will do) the suspect file and send it to virus @ avast.com (no spaces). They can check and see why it wasn’t detected and possibly consider the addition of those packers if not already supported.
Give a brief outline of the problem (possibly a link to this thread), the fact that you believe it to be a either a new, undetected virus or false positive and include the password in the body of the email. Some info on the avast version and VPS number (see about avast {right click avast icon}) will also help.