How to leverage Record Type / Transaction Type fields from scripts?

I'm trying to build a custom record that describes how to pull multiple record types together for generating documents. That is, I'm using TemplateRenderer.addRecord() to accumulate what I need passed to a template. Fairly simple, or so I thought.

I tried using a Record Type field to determine the type of the record and, in some cases, get the scriptid for use as the type with record.load(), but that appears limited to custom records, as other types I've tried just give an empty string. I mostly care about transactions, though, so for now I thought I would try using a Transaction Type field, but trying to pull a scriptid causes an error, and I can't seem to pull any joined data whatsoever. As it stands, the best I can do is apparently creating a Free-Form Text field to store the record type for loading, but this is sub-optimal. I'm left wondering about the usefulness of Record Type & Transaction Type fields.