I think you might have misunderstood me.
SCI Companion knows where all the text (be it words you looked for or spelling mistakes) come from. That's why you can double click lines in the output panel, right? A plaintext file containing only the raw text does not include anything about where that text came from, and adding that information introduces things that should not be proofed. Also, importing the changes is not as easy as you might think, especially with the way the script strings are found and exported because those are done quite differently from how "find" does it.
So besides the script strings being Weird...
Non-interactive - already hard enough I'd say.
1) Load up a hidden ASpell instance and a pipe.
2) Iterate through all text and message resources.
3) Pass each line to ASpell, read back its judgment.
4) If it's not "*", put a clickable line in the output panel.
5) Let the user click each line and apply each fix as they see fit.
Interactive - would need creation of an all-new dialog box and holy shit this is MFC and oh god oh god
4) If it's not "*", pop up that dialog box, with the original line and ASpell's suggestions in it, let the user judge or edit it, apply the fix.
External - can't be plain .txt but who knows, would require some kind of re-import feature
1) Hope source data is available for the script strings.
2) Iterate through all text and message resources.
3) Write each line to a specially-designed file marked with its source.
4) Let an external tool work with only the "text" fields.
5) Re-import everything, somehow.
...yeah. And then there's things like words it simply doesn't know that need to be added to a personal dictionary, which the non-interactive option can't really do. (It's done by sending a special command line to ASpell.)