Did you ever find out about this? I'm curious as well. I'm pretty sure the 3 B+ is still HDMI v1.3a but my understanding is that the HDR metadata is baked into a 444 signal; it just exists outside the visual frame. And since this isn't trying to send real HDR video content, the full high res data rate isn't needed. Just sending a single pattern would only need a very small refresh rate, and therefore, a very small data throughput, so I'm wondering it the 3 B+ would still be capable. I think the minimum for HDR10 only needs 1080p at 60hz with a data rate of 4 Gb/s while non-HDR at 1080p/60hz only needs 3.2 Gb/s. So I assume the extra 0.8 Gb/s has something to do with the extra metadata. Regardless, that's well within the pi's HDMI v1.3a capability which is 8.16 Gb/s.
I'm imaging something like the HDR metadata existing in the space between a physical resolution and a virtual resolution as it's piped to a raw frame buffer.