Hi Peter
I suggest contacting the manufacturer - surely paint over their product is a usual practice.

Full cure times, a good wash to - perhaps - remove any residues, and some sanding are all steps I take when painting over something. It's unfortunate that ZAR is so expensive there, as it has adhered very well to my PL premium test pieces. Perhaps some other varnish? Your testing is a good start, in any case.

BTW a few of my friends are joining that trip down the Loire - Michael Storer, and my neighbors Paul and Sharon Helbert. Sounds like a fun time!

Good luck!
Dave