Hi.
Maybe I don't have the proper answer to your problem, but it could be an interesting thing to use a tvheadend instance (on the master) as iptv source for another tvheadend instance (as slave). I tried quickly, but didn't find the proper format for the configuration.
One of the cool things in tvheadend is merging sources to channels. So with this you could setup a channel on your htpc which consist of a local dvb service and a iptv source (which may come from the tvheadend on the NAS). So tvheadend is the software used to share dvb devices within a network.
I'll see if I find out more in the tvheadend source, if this setup would work.
Another solution for you, if you can completely separate livetv from recordings, are two independent tvheadend instances. xbmc pvr on the htpc accesses the local tvheadend for livetv and uses some nfs/smb share on the NAS there the recordings are created.
Both solutions have the disadvantage, that pressing the "record" button out of livetv need some workaround, since your livetv uses the local tvheadend, it will also record local. in this case you may need to record to a share on the NAS.
Personally, I'm also interessted running tvheadend on a NAS with USB DVB-S2. Now I have a full blown server, which is not necessary and consumes a lot of energy (3w idle would be a dream). But I'm thinking more to have all DVB devices on the NAS to profit from sharing all the devices between the clients and the recordings.
Do you have already tried to run tvheadend on your NAS accessing DVB-S via usb? How much power does your USB DVB-S device consume if not used?
Ciao,
Martin