If this is possible, then device makers should do it, ship the free-software firmware on the device, and then get it RYF-certified. No blobs needed, ROM or otherwise! Problem solved!
It seems like a feature, not a bug, that the RYF certification process makes it more painful and expensive to release devices that rely on proprietary software.
The device maker could ship with programmable proprietary firmware and have the possibility of being RYF-certified in the future if someone writes free firmware. Or they could ship with proprietary firmware in ROM and be guaranteed RYF certification immediately. The rules encourage them to pick the second option, and it's no surprise that companies such as Purism have done so.
Yet that option is never better for user freedom, since making software un-upgradeable does not make it any more free. And occasionally it is worse, in the event free firmware becomes available later on.
The alternatives to this policy are allowing all blobs, in which case the RYF certification isn't actually verifying anything, or not allowing any blobs, in which case RYF cannot certify any devices made after 2009 (something the author also takes issue with). Making it expensive and risky for device makers to rely on blobs is the only middle ground that makes sense.
In many cases they don't even have them or may not be aware that there's an upgradable firmware in some of the components they're using at all.
The other point they ignore that if some part is programmable but currently requires proprietary firmware, it's possible (and this has happened) that people reverse-engineer it and produce free software that runs on it. BUt if you bought the FSF-blessed version of that device you're then stuck with the proprietary version of that program forever, and worse, you can't get any updates for that program. You can't get a security fix, and you can't replace it with the free program.