Usually, what we register using cot, used the register name whose domain is contenttweaker.
How about a custom register name, which enable modder put a ResourceLocation instead of a String to expand the domains of register names.

We deliberately made it so that all blocks are registered under the contenttweaker namespace.

Reason for that is so that nobody registers items under another mods' namespace.
This is so that nobody raises an issue at another mod for items that were added via ContentTweaker.

Can you give a bit more context of why the modid is important in your usecase?

Under normal circumstances, it is enough to use contenttweaker, but sometimes we may want some personalized content or traceability, especially Custom load source or In Pack script, although CrT did not implement these two in 1.12.2.

I made a Mod to implement these two parts, and customized domain is also a normal requirement. Oh, is just a hope. If this is not considered Cot's responsibility or plan. You could close it.