In tube amps, loading like this won't do any harm, just a little more distortion quicker. It does not risk fire, flame, letting the magic smoke out and fried tweeters like in SS.
Convential tube amps have output transformers. The loading on one side is reflected to the other side. It's always a ratio from in to out. They're designed for an anticipated load, it's not an impedance that they present. So when you change the load on the speaker side, it reflects this change to the tube load. As you know, speakers load changes by frequency anyway. All over! So as you suspect, a ribbon's uniform load has some advantages in that it will remain at the spec and not swing all over.
By presenting a stiffer load you'll be moving the tube a bit away from it's swe-etest spot, which Bob would have presumably design the amp to run in. Stiffer load means higher power demanded and higher distortion, lighter load means lower power, less distortion.
The more traditional output xfrm loading is to use about a 7.6K load or so for a PP EL84. Bob opted for what he thought sounded nicer and went with 10K OPT. So I'd suspect you'll move a bit off the swe-etest spot, but probably not be loaded so, so different from what's more typical on the EL84.
So have no fear, just be aware that as you push up in level, the waveform will start to flatten out as the amp runs out of ability to go there. It doesn't tax the amp though in a way that causes harm...just can reach the point where it gets less pleasing.
That's the word, until you get the official word...