Sure -- almost all the buildbots run in VMs, after all :> It runs VMWare, I believe Ryan has said.
The ports that are cpu specific are generally forced to build locally. I use VirtualBox for 10.4 Intel and 10.6 32bit (and a bunch of older Windows systems -- windows95, windows98, windows2000, etc). I use Parallels for 10.5 - 10.14. For 10.5 and 10.6 you need server editions of MacOS, as mentioned. I use qemu for 10.4 PPC and 10.5 PPC on a MacPro 5,1, but that is really stretching it I guess .... To share local archives, I do this: https://trac.macports.org/wiki/howto/ShareArchives2 I have a huge library of > 800 ports built for 10.4 PPC, and more than that built for 10.5 Intel and 10.6/libc++ shared locally. Ken