One that came up, is that I could use VirtualBox (or VMware Fusion) to have a virtual machine running Windows, in which I could run Warcraft and ISBoxer.
Note1: I'm not interested in using BootCamp as I don't want to split my hard drive space.
If you don't want to split your hard drive space, I don't think you can avoid that by using a VM to solve the problem?
Could this setup work? If not, what alternatives could I look at?
It should technically work, assuming your VM can provide the performance you're looking for. Boot Camp obviously is an option that I believe most Mac users around here are familiar with and probably have tried. Parallels may also work? I know less about that option myself to say.
If it's possible to run it using a separate external hard drive for the Windows partition, then this option could perhaps work, but I'm not sure if that's even possible or if the performance would work for multiboxing.
I'm no expert on Boot Camp in particular, but if it's anything like other similar software it should let you use basically whatever drive you want. Certainly you can use an eSata drive (Macs have this sort of thing, right?), even an SSD, and perform just fine.