I think that was a choice. He mentioned that he built a four level video amplifier to replace the stock amplifier. That's the card hanging on the neck of the CRT. [nitpick]The video capabilities of the original Mac design are 1-bit, not 2-bit. [/nitpick]
Examining the video amplifier schematic for the original SE/30 and comparing it to the schematic for the Micron adapter helps to make sense of the thing. As viletim mentions, the original just has a switching transistor as the amplifier. It is on or it is off. The Micron adapter has considerably more flexibility.
I do not think that the results would be good. I think that efforts to convert the VGA signal to a compact Mac signal are (mostly) doomed to failure. What viletim did was to essentially create a custom video output which really wasn't VGA any more. He just used a PC VGA video card as his hardware. And that hardware happened to be versatile enough to let him do it. But notice that he used software to modify the timing and the size output of his display to match the expectations of the Mac compact CRT.
To do something analogous on a Mac Mini, one would either need to find a way to hack the software parameters of the display output to get the smaller resolution, or get something like a cube motherboard and install a hackable or custom built video card. I don't think that 60 Hz will be a problem, because many LCDs seem to like 60 Hz, so the Mini probably supports that. Getting the smaller resolution and going grayscale could be a challenge, but surely all that stuff has been solved by folks who write utilities such as SwitchRes?
Wait, isn't there a PCI-e connector of some kind on the new Mini motherboards holding the wireless enet card? That could be hijacked for custom video output if necessary.