I did some further fiddling with the paper developer, and finally got something that works for me.
I started by adding 1 tbsp borax and 1 tsp lye to a liter of water, as recommended by Patrick. I added 2 tbsps ascorbate to that, twice as much as I've used before. In terms of concentration, that's a pretty strong developer!
The large amount of ascorbate solved the problem of aerial exhaustion. The developer now lasted hours and hours in the tray. It also gave a very good maximum black, even without phenidone.
However, there was another problem: image density depended on the time in the developer. Unlike a proper developer, the prints got darker and contrastier with time, even after the shadows reached maximum density. In other words, I had to time the prints in the bath.
Disappointed! After observing that ascorbate and carbonate developed paper without phenidone, I had the ambition to make a good two-component developer. But time-dependent development isn't acceptable.
So I gave up the two-component idea. For the next batch, I added phenidone (8 ml of 0.8% solution) and 1.5 tsp bromide. That fixed the timing problem.
After about 4 hours, the phenidone was exhausted, which meant that the highlights took more time to appear and contrast crept up. Adding another 4 ml of solution fixed the problem.
The developer now gives contrast, hue and maximum black that are a close match for Neutol WA, without the hydroquinone, so I guess I should be happy. I have no idea about storage life, though.