Bill, not a solution, more a bodge.
Take @Chris's suggestion of hiding WB elements by totally hiding things at the FlexGroup level (or all content in a Freeform layout), on the Variant page.
Then set the Variant Properties' Width to 12800px so the cut is at 7040px, way wider than most screens can show [Yes, you could set the browser zoom to 20%].
The Variant content does not get published.
The saner approach is to work with WBs on both until Main is complete.
You then clone your design file so you have XYZ-Main.xar and XYZ-Variant.xar.
Mangle XYZ-Variant around after changing Main's Width down to your required 'variant' width.
Finally, copy page-by-page from the Main design of XYZ-Main into a non-WB design and do the same from the design of XYZ-Variant.
I'm hoping Xara gets the clue that WBs are problematic.
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