creating a custom field for transaction line i chose Free-form Text; however, turns out we need a Long Text field, but that option is not available in the Type dropdown . . . is this field keyed to certain entity types or system-level switch of some kind?

Text Area is the best you’re going to get for a single file in the Transaction Line record. If you need more than 4K characters, maybe consider using multiple Text Area fields, or use a Document field and link to a flat file in your File Cabinet.

ok . . . i couldn't find the docs that say "Long Text not available for Transaction Line" but apparently that's the case . . . luckily Text Area is going to work 🙂