CPH-Flyer - thanks for all the help in this thread.
Can report very smooth HND transit on separate tickets (though likely helped along as all on NH).
Origin airport were able to issue boarding passes for connecting flights and through-check bags.
At HND we were escorted from the plane to transit security by gate 112, and within 10 mins were back in the lounge.
Plenty of other people camping around the terminal and lounge also doing airside transit.
Most shops and restaurants are still closed. Duty free closed at 1700. A few outlets at the food court near gate 114 were open until 2200.

However of note: transit security wanted to see an 'international transit pass' (a green laminated card that transiently changed hands from airport agent to NH agent to us then back again) and scan our onward boarding pass. The airline connections desks were situated after the transit security desk, and were completely deserted (I don't doubt that they can summon a representative if needed though). I therefore imagine you may come up against a bit more friction if you aren't able to check in and get boarding passes for your connecting flight in advance.