Override synctex position

The design itself of SyncTeX makes what you're asking impossible: it hooks at the level where lists are transformed into boxes and where boxes are shipped out for output, with no influence whatsoever on the workings of TeX. In particular, its output file cannot be modified by \write, because it doesn't correspond to any TeX output streams.

You can read Jerôme Laurens paper on TUGboat describing the system and better explaining how it works than I can do here.

What you want might be possible with pdfsync that uses a different model, that is, inserting whatsits in the lists TeX is making. However, this package is obsolete and has many problems that make it moderately useful only during drafting of a document, not for the final typesetting, because the whatsits influence, often very badly, box alignment and production.