Thanks @dianemercier! I'd love to get as much feedback as possible!
I'm still researching this. As far I can tell, the first critical step is to get MARC data into a tabular format (e.g. CSV, Excel) using something like MarcEdit. One can find the way to do this in MarcEdit in the MARC Utilities -> Export Delimited window. At this point, one can choose which (sub)fields they want in the CSV file.
For each (sub)field, we can probably define a good JSON Table Schema constraint and type for validation. For instance, we can define 043$c to be limited to ISO codes. For (sub)fields with multiple values embedded that might be more difficult. At this point, when the data is in tabular form, we can do the validation. Once the data is validated, you can then use an Excel import tool (e.g. http://manual.koha-community.org/3.2/en/marceditexcel.html) to get the data into software like Koha.