record.SubmitFields

to the Item Record? to set a field value with an Image Internal ID?

Well, I'm getting the list of associated images from the item record. It would be looping through that image list of IDs (which I'm already doing for other purposes anyway) and submitting the fields to the Document record (ideally). Unless there's another way to bulk/programmatically update image alt tags on the Document record...

I'm not fully understanding what you are trying to explain, could you show some Screenshot? what is the Document Record? Those Images are in the file cabinet? from where in the Item Record are you reading them?