I've now tested this and am happy to say that I've found a solution that works for me. (Thanks for the GEDCOM hint in your reply, kj.) FTM2012 does still have an issue though:
- The problem happens when uploading trees from the latest FTM2012 version to the Ancestry site. It didn't happen in FTM2011, which didn't offer TreeSync.
- Click on the 'Upload and Link to Ancestry...' button on the 'Plan' screen. The 'Upload to Ancestry' pop-up then appears.
- If I leave the 'Upload without linking' unchecked then all characters show up fine in the resulting tree on Ancestry.
- If I however check 'Upload without linking' (screenshot) then all foreign characters show up as '?' in the Ancestry tree. It doesn't seem to matter which other checkboxes are checked/unchecked, the characters always get mangled. It seems this is out of the user's control.
The fix however is to export the tree to a GEDCOM 5.5 file (File->Export->Output format=GEDCOM 5.5), stick with the default settings Destination="FTM 16" and Character set="ANSI"). Then upload the resulting file from the Ancestry site (Family Trees->Upload a GEDCOM). Slightly more cumbersome but FTM now does what I want it to do and displays all foreign characters properly on the Ancestry site.