It works 
:
(Well, mostly. 1, 2 and 4 boilers work as expected, but 3 boilers only power 2 centrifuges for some reason - swapping out for an HV motor driven by 3 MV batteries works fine so the problem is at the turbine.)
It’s quite hypnotic to watch, but I don’t have the energy to put together a gif right now.
The variable turbine driving it all is mostly an exercise in proving to myself that it can be done, but in reality it’s more trouble than the efficiency over electricity is worth - there’s more infrastructure beneath the floor, and the igniters are a little temperamental in a way that would require the whole system to be at least 2m longer or wider to fix. That’s assuming you want the Gibbl-O-Meter-driven boiler-pre-heat option permanently turned on - it gets even larger if you want to be able to turn it off completely. Without pre-heat, the boiler response is just too slow.
To make best use of the transforming gearboxes for now, it’s probably best to carefully match machine speeds along a whole production line and have the whole line either on or off. One feature that would help with that would be some way to limit how many packets a gearbox outputs to an attached machine - right now if you replace the first bronze centrifuge with a steel centrifuge the rest of the line stalls as it takes the full 256 RU/t. If I wanted to put 128 RU/t into the first machine and 64 RU/t each into two others, currently there isn’t an easy way to do it. (I tried selector covers/tags both with and without universal extenders, and none worked. Selector tags feel the most appropriate, I think.)
Eventually, to really make use of these new gearboxes, some variability in steam generation would be good. Ideally, (and I’m dreaming again now) make the current boiler ratings a maximum with steam output scaling linearly with HU/t below that, and allow us to attach redstone selector covers to burning boxes (gas only?) to limit their fuel usage and HU/t output to some fraction of their maximum. It’s not totally unrealistic and, as it’s explicitly making machines less powerful, it can’t be accused of being OP.