I've been trying to use circuit conditions to only produce certain products when I am short on that ingredient, but there doesn't seem to be a way to set an assembler's ingredients to basic ore sorting, only advanced ore sorting. Is this a bug or just an oversight?

I believe you should need only give it an iron ore signal, although I'm afk at the moment. I'll look into it further when I've time.

I tried that, but it doesn't seem to work, it does not set the recipe to anything with an iron ore input

You can use the pipet tool in the assembler on the iron recipe (the basic sorting), exit the assembler and enter the combinator.

I see the issue now: I hadn't set a constant output. Thanks for the help.