The concentrate works! Here are the two curves of XTOL versus concentrate:
As you can see, they're about the same as mixing ingredients separately, and the dev closely follows XTOL. Grain is the same. So dissolving everything into propylene glycol didn't hurt anything, despite a little scare I had when the liquid turned orange and clear again because I dissolved things in a different order. I suggest following the order below:

Propylene glycol .................... 25 ml
Sodium metaborate 4 mol ..... 6.7 g (dissolves in about 3 minutes)
Ascorbic acid ......................... 8.5 g (dissolves in 7-10 minutes)
DimezoneS/Phenidone .......... 0.2 / 0.105 g (dissolves instantly if ground in mortar-and-pestle)
Propylene glycol to ................ 33.3 ml (final volume)

Heat to 90C to dissolve everything and drive the water out of the metaborate.
To make one litre of developer, mix 33.3 ml of concentrate into water containing 90 grams of sodium sulfite. That's 1+29 dilution.
Times are same as XTOL. Target pH is 8.33.

The only change to the formula above was to start with 25 ml of PG instead of 24 because the powders expand the liquid less than I expected, so you can start with a bit more.

Whew! This has been a long project, and I still need to run tests on sharpness and different kinds of film. But this is looking good.

@Alan Johnson: This concentrate is similar to D316, except for adding more metaborate to boost pH. So the keeping-test you're doing will be helpful for this as well.

Mark Overton