As I recall, that is basically the case for the OPF files, that they’re simply TTF files with glyphs that all have open paths. You might be able to produce them by exporting as TTF (making sure Remove Overlap is unchecked) and renaming the resultant .ttf file to have the .opf extension. To export as TTF, select the OTF icon of File > Export… (Command-E) and then select the Save as TTF checkbox.
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