Variant models, where you don't know the all the model variants in advance

I am looking for a way to define model variants, where you don't have the list of possible variants in advance.

Rationale: I want end users for my blockset library to be able to define their own variants, without having to manually edit the library to set up their own variants inside of 100+ blocks. For that matter, I'd like to be able to do this. Just about every block in my blockset needs to do some custom low-level calculations.

I know several hacky ways to do this, like chaging the ReferenceBlock parameter within the block's MaskInit. But I'd like to avoid those. Suggestions for non-hacky ways to do this that don't involve variants are welcome.

In another case like this, I was able to use Two-Way Connectors to attach a block as a sort of a plugin via a PMIO port. This was actually a really spiffy solution for that case because the low-level custom calculations were only used occasionally, but for my use case here, where nearly every block needs it, that would be extremely unwieldy.