Even with the desktop I want to see “ECC guaranteed” on the box, not a pile of parts that “should” work.

It's hard to get more ECC guaranteed than the mobo manufacturer having ECC ram on their QVL (qualified vendor list) for the motherboard and indicating that ECC is supported in the specs. I suppose you can just get a complete workstation with ECC from HP or DELL and that'll be guaranteed to work but you'll certainly pay a hefty premium for that.