Ok, wanting structured data is a decent argument, but why this random arbitrary case in particular? There are hundreds of fields like this that people might want to use.
If we're going to support one random cosmetic field, we might as well support them all with a generic structured data format.
I'd rather we just didn't have this essentially pointless "feature" at all. Let's try and keep as much cruft as possible out of the payment protocol. The textual memo field is already more than sufficient.
I think there is nothing wrong with having a numeric memo field, which
is effectively what this is. Structured rather than unstructured
data.