thanks for you reply, I really appretiate your interest in the product. Unfortunatelly, no recent version of avast supports older flavors of WHS.
If you really require the features you have described I see you have several options on the path that might want to take. Please note that I am not discussing the economics of either choice, just trying to give you hints as to what the options are.
1) Get the avast SOA product, http://www.avast.com/en-us/small-office
. While I am not aware that this was ever tested in our QA to work with WHS, it will five you even greater control over your network (but no integration into the WHS infrastructure on the other hand...) Thinking about it a while longer, this looks like an interesting test case for our QA and I will make sure such test is executed during the next week.
2) Another option would be to upgrade to WHS 2011 and use a third party software, such as SbaleBit's DrivePool (plenty other plugins around that do similar stuff). This will allow you to set up drive pools and "single directory, no drives" file structure.
3) The last one is more of a "techie" solution... You may also set up a RAID on your WHS. WHS 2011 dashboard does not contain means to do so, but if you know what a RAID is, I'm sure you'll know how to get one up and running. :-) I do admit this is somewhat harder to implement and may require investments in special hardware, on the other hand, such solution will give you fail-safe storage for your valuable data... In fact, I have RAID 0 set up at home to keep all of my important data (that I never use anyway... ;-))
I am sorry I could not give you a better answer at the moment, but I hope at least one of the three above stated hints will help you make a correct decision.