There already is an option to exclude files from scanning, but given what you say that it might be infected later (as far as avast is concerned it is already infected).
You need to submit the file to
VirusTotal - Multi engine on-line virus scanner I feel virustotal is the better option as it uses the windows version of avast (more packers supported) and there are currently 32 different scanners.
Or
Jotti - Multi engine on-line virus scanner if any other scanners here detect them it is less likely to be a false positive.
Whichever scanner you use, you can't do this with the file in the chest, you will need to move it out. If avast is the only AV that detects it then submit it to avast as a false positive, Also see
(Mini Sticky) False Positives, how to report it to avast! and what to do to exclude them until the problem is corrected. This would correct any FP in the VPS and allow for the file to be scanned without problem, unless it became infected at some point in the future.
If multiple scanners detect it my thought would be that there must be other applications out there without this issue able to do the same task.
The other issue is how is avast or the user to know that this infection is different to the last infection ?
So if this were implemented (and somehow I doubt it) it could have a very high potential to harm your system if you got your decision wrong (to allow it to execute) and I guess avast would then get the blame. I as a mere avast user feel this could be a very dangerous option.