it would be possible to have a VM on the NAS and use it with Fusion or Parallels; but then again performance might be too slow for any great experience.
Do you mean use one of the HDDs as the sole HDD for all Win system and data files ? If so, then Parallels might run OK if you use this as the mapped C drive, but I doubt it. It would run OK if you kept the C drive as a 16GB drive (or smaller) on the MBA and all other stuff on a mapped drive on the NAS as a drive within your user shares. I used to do this all the time with Win7 (to save space on my system SSD). But it only worked OK if there was a LAN link to the NAS not a WiFi link. If the NAS supports iSCSI then you can use this as an alternative to a mapped drive.