I have made quite a few Delrin venturis but they've all been the old nva through type.

Can I tap Delrin to use the newer single sided nvas more common these days.

Yes

What thread is used on most r/c type needle valve bodies?

Control line venturi and needle valve on the .15. A bit over 5mm bore. It needs muffler pressure or bladder feed.
I just checked a Fora needle valve that is 4mm x .5mm pitch. For control line needle valves I will just use SAE threads like an 8-32 or 6-32 nipple, as the LA .15 has been done after the plastic broke, just because I have those taps and dies handier.Normally I will go to a remote needle and just thread in a home made nipple into the venturi. These are some poor examples of repairs, and the Picco that use a remote needle.