I like the idea of using the flip dots as indicators. They are such cool displays ;)

Yes, indeed !

I have only 4 so I must choose wisely.

OTOH I have some hundreds of "Glühbirnchen" (tiny incandescent lamps) which would be awesome, if only they didn't draw 50mA@3V. Displaying the whole register set's values in binary would consume 12A/ or about 40W...

Now that I think of it, maybe I have to use one flip dot array to display the register set, but I have only 2 arrays that I dedicated to the memory, and the dimensions are not suitable (16×28, I need 18×16).

I also consider the Lixie approach or something similar...

Oh yes, the Lixie is very cool. And very affordable. It has a modern look though (at least to me).   If you want that, cool, but I know some people want a retro/nixie look.  It's all the experience of inventing and learning though, isn't it?

Nixies would be interesting though I have doubts about practicality... And I have not seen hexadecimal Nixies yet (and I don't want to do octal :-D)

The look is only one part though. I try to design a sort of playground that is pretty safe, so no high voltages or temperatures.

For now I ponder about the practicality of doing a quadruple Hex=>7 segments decoder to display the Imm16 field :-D

>And I have not seen hexadecimal Nixies yet (and I don't want to do octal :-D)
I have a twenty 14 segments +comma/dot display.  It was from an old pinball machine.

@K.C. Lee I maybe have used the wrong word then, I was thinking about stacked digits

https://en.wikipedia.org/wiki/Nixie_tube

Or something like this

or 

https://www.tindie.com/products/connornishijima/lixie-an-led-alternative-to-the-nixie-tube/

I was not thinking about VFD :-)

It is the technology not the form.

For the form and technology, I will let common sense (parts count and power consumption) decide.

I'm not sure of the right trade-off :

* 7-segments light several segments at the same time but require less segments.

* unary require 2× more display points but only one at a time

I can deal with diode matrices, though I'll need to wire 4 of them :-D

Make a backlit wheel of digits out of a PCB driven by modified relay with ratchet.  It would fit in better with the electro-mechanical theme.

Hahahaha :-)

I had considered making a crude 4-bits DAC and labeling a needle ammeter, or something like that...

Wonder if these could be made DIY at home, perhaps larger.

more or less, the principle is pretty simple :-)

* find a permanent magnetic material, make a very thin and lightweight "dot"

* find hysteretic material, some ceramic/iron/ferrite/whatever and make a U

* wind a magnetic coil around the U so you can control the magnetic field in the ferrite

* pass some current in the coil to reverse the stored magnetic field => the magnet will align and flip !

Yeah, thinking along those lines, more worried about building something like that with my fat fingers :-) might have a go with it at that :-)