I had an idea and it turns out there is an off-the-shelf part that exists:

Coincidentally, it seems that M6 is the only option that even has a flange substantial enough to work. 

Instead of having to carefully tap stainless steel threads for 90 minutes and still have them slightly misaligned, the idea is to have appropriately sized holes in a steel plate and the insert flange sandwiched between the plate and the heatblock. The insert is somewhat tall but I feel that it will be quick work to file them down.

As you may see from the image, the minimum spacing between inputs will grow to 14mm, but it's probably for the best. There's a bit more heatsink / bowden coupler options available when the spacing is above 12mm. For context, the Coaxial8or currently has 10mm spacing.

One of the goals for the next design will be to have the 8 inputs converge as near as possible, hopefully so that changes are faster. Thus, the input pattern may follow the Kobra 3 Filament Hub:

The aspect ratio is somewhat squarish though, which might not be ideal: