The subsetmerger operation has support for local files. Basically, you have to hit this code path: https://salsa.debian.org/fonts-team/gftools/-/blame/debian/0.9.991+dfsg-2/Lib/gftools/subsetmerger.py?ref_type=tags#L403
You should achieve this by having "../subset-files/notofonts/latin-greek-cyrillic/main/sources/NotoSans.glyphspackage" available.
