Considering fitting a HRC RVF CDI & loom to my NC30, is it possible to retain the NC30 tacho given the different number of pick-ups?

No mate, you need the rvf tacho also. As well as the rvf single ignition pickup, starter clutch and more.

If you're just doing it to be able to use the hrc cdi then I'd say forget it, not worth the hassle. Fitting an ignitech aftermarket ignition to the nc30 loom would be easier and cheaper.

Good info here: http://400greybike.com/forum/viewtopic.php?f=24&t=33390