So I switched to simply running all the processes (master, meta, chunkserver) on the same hardware and got rid of all the VM's. That worked fine. I defined a separate network - currently fully confined to the same host - in order to host the MooseFS installation. And MooseFS clients have started to run just fine. I got a performance of up to 80 MB/s for reading data from the MooseFS over a 1 Gbit/s network.
However, one problem remained. Running UNFSD on the same machine I got very poor performance. As few as 5 clients could drive it down to just 30 KB/s! And that on an 8-core 48 GB RAM machine - while a MooseFS client would read at 3 orders of magnitude as much!
Surprisingly, the fix was simple: if I ran UNFSD on a separate physical machine the performance went back into the tens of MB/s range. So that was what I settled for. That NFS server machine is currently just a CentOS Linus MooseFS client sitting on the "general" network - different from the one hosting MooseFS - and sporting a mere 2 cores and 2 GB of RAM. So I guess for now I have a working solution.