This is possible even for packed mode.
The way I see this could be implemented:

1. Allow any circuit-categorized entity to be built in the editor
2. Create a single infinite surface
3. When the processor is packed, transform all entities into their respective packed versions (when possible)
4. Move those without the packed version to another surface and rewire them to new entities
5. Print warning that this feature is EXPERIMENTAL and something may not work as intended, which could (and would) be disabled in settings

This would be slightly less performant than truly packed but infinitely more useful.

Sorry i am a bit busy, nowadays. The idea is interesting. I am thinking of something of something more simple. Just allow any kind of combinator but in this case disable the switch to packed mode. I just wonder what is the memory footprint of having a surface by compackt circuit and if it is worth to gather all unpacked circuit on a single surface.

Maybe you can try the new settings 'allow externals combinator'. The processors that contain an external component work only in non packed mode.

That's pretty cool, certainly seems to work, I've used various unsupported mod components in a circuit. I tend to make fairly heavy use of some of the features in packed mode, e.g. external displays so it's not a full solution, but I love the extra flexibility it gives. Thanks for your work on this Telkine2018.