As far as I know UA doesn't block NH award seats (at least on international routes; not so sure about domestic). However there seems to be a glitch in UA's search engine where NH segment only shows up as part of a multi-segment itinerary, not as direct. For example, even if NRT-SFO is available, it doesn't show up when you search NRT-SFO. But it shows up when you search NRT-LAX as the first segment of NRT-SFO-LAX.

You can also double check on NH website or KVS tool (you have to be an AMC member).