@John I found it!
There’s a term I’ve been looking for: array uniforms.
I knew there was a forum post where we’d discussed this before, and there was a term I couldn’t remember from it:
In this you mention something called array uniforms and you seem to think it’s possible that they could be implemented in Craft shaders.
It seems like they could be used for cool things—are they also not possible for the reasons you mentioned?