Exterior foam with thermalbuck and European windows- no wood to attach window flashing to!

This is a reader question posted to a building forum, not news you need to act on, but it's worth understanding if you're ever planning a similar renovation.

A homeowner is finishing a post-and-beam barn using exterior rigid foam insulation with a product called Thermalbuck (a foam window buck system that lets windows sit out past the wall's wood framing) and European-style uPVC windows from the brand Drutex. The wall assembly uses a weather-resistant barrier (a sheet that keeps water out while letting the wall dry) called Thermaltight, taped with Contega IQ tape, plus a rain screen (wood strapping that creates a gap behind the siding so water can drain and air can circulate) under vertical corrugated metal panels.

The problem: the windows sit close to the edge of the foam buck, and the sloped sill meant to shed water away from the house isn't angled enough on several windows to do its job. Worse, there's no solid wood left near the window to nail or screw the metal flashing into, since the foam and buck system replaced the usual wood sheathing in that area. The homeowner is asking whether adding a strip of wood over the buck, after the waterproofing tape, would let them build a steeper slope and give the flashing something solid to fasten to, and whether there's a way to flash the sill without using fasteners at all.

There's no rebate or program news here — just a technical installation question about window flashing details on a foam-insulated wall.