As an alternative, you might consider letting Hypercard itself open the text files and 'set the script of' as needed.
Yup, that would have been easier. It's been decades since I've done anything with HyperCard. I had to re-take the built-in intro course again :)
Would that overcome the size limit?
Does HyperCard implement its on text handling for the HyperTalk editor that doesn't rely on the TextEdit toolbox service (which IIRC is the source of SimpleText's 32 kB limit)?