Why not? I mean why isn't it possible? It is currently impossible to install the USB driver on x64 versions of windows, due to signing issues, but you only want to use the network client part, so I fail to see how that would be a problem. You'll just have to deal with everything from the command line.
Hyper-V Server 2008 R2 is just a Windows Server 2008 R2 Core install with all roles but Hyper-V server disabled, some helper scripts, and the odd funky behavior here and there. If you say it is not possible from a licencing point of view, we talked with MS at this one place where I work, and they explicitly told us that things like backup agents, extended hardware drivers (HP OpenView agents, etc) and somesuch were not considered "usage", and were perfectly fine.
In fact PowerChute Network Shutdown explicitly supports Hyper-V Server 2008 and Windows Server 2008 Core with Hyper-V role installs.
I don't have any Core installs available in my lab to try it, but I'm fairly confident it would work, apcupsd has very little windows-centric requirements, it is just a service, which should in theory work. If not, a bit of hacking could make it work.
If you report it does fail to work, I will setup a test vm with a Core install and give it a try, for kicks.