So I've listened to the entire series of Deckingman's multi-material series and the 2 things I remember from it are that:

1. PLA hydrolizes in under 30 minutes using an off-the-shelf X-in-1-out hotend and between 30 minutes and 1 hour in his design.
2. Materials that one may want to mix together (e.g. ABS + TPU)  will likely result in very poor interlayer adhesion 

TPU coating?

Now, I'm thinking that because this is a coaxial hotend and interlayer adhesion is predominantly a surface-to-surface interaction, I could possibly have a transparent, flexible filament like TPU as the final coat. It shouldn't slow down the print speed as only small quantities would be needed, and TPU has one of the best interlayer adhesion properties of printable filaments, as well as bed adhesion to PEI. 

To put it another way, this is the "glue" or "motar" filament. It also hopefully keeps the interlayer adhesion consistent even amongst different colours of the same material type.

If colour (and flexibility) is of little importance to the application, PBT filament may be ideal due to it's allegedly high Tg, stiffness, interlayer bonding and low required bed temperature.

Cleaning filament

Regarding the first point, I'm thinking that the cleaning filament should be the first input, allowing the printer to clear out the center column of the coaxial block as well as make material changes easier.

Park and reprime

Additionally, I'm thinking that the most straightforward solution to prevent potential issues is to just park the extruder every 40 minutes or so and fully reprime the hotend. The most optimum way would be to only purge the inputs that haven't been used frequently, such as any filament that hasn't been used for at least 5-10% of the duration between the last purge and now.

The rest of the lineup

I was partially wondering what I'd be using 8 inputs for, but now that I've thought about it, it's barely enough. This would be the lineup for full colour and alpha channels:

1. Cleaning
2. Transparent PETG
3. White PETG
4. Yellow PETG
5. Cyan PETG
6. Magenta PETG
   1. I've ordered 2 - 6 from most to least sensitivity to the human eye. Less sensitivity equates to less preception that a colour is right or wrong. I'd imagine it's a lot easier to see a small streak of yellow on white than a small streak of yellow on magenta.
7. Optional / Speciality (e.g. Galaxy Black, Copper, Marble, Soluable)
   1. If black isn't used, the 3D print is likely going to have the same kinds of "blacks" as an LCD screen, which is just a dark grey. OLED inky blacks will need a black filament here.
8. Glue (Likely TPU)

Conclusion

I think it's a good enough strategy to try, and I've modelled in a variety of improvements and discoveries over the years from the community to at least have some indication that this hotend would improve on the work. 

Unfortunately, like some 2D colour printers I hear about on the internet, you actually won't be able to successfully print if you're out of cyan yet just want a B+W 3D print 🤭😅 (or something in that input).