Why do you need DOS 5.0 (and why 5.0 specifically)? It's theoretically possible to do this, but you'll need to have some sort of bootable media that the Mini can read to do it. I read somewhere that the new Duos can boot from a USB drive, so a 1GB USB drive would probably be ideal for this. There are all sorts of "how to" articles online about making a USB drive bootable for Windows and some DOS versions.
As for booting from the NAS, I'm kind of tending toward (see the hedging here?) the idea that it wouldn't work. Network booting usually requires a specific boot image on the network somewhere, and it's something that's not the easiest thing to set up and manage. And most important, I don't know if the network adapter in the Mini is "bootable."