Is it viable for devs to implement heatmaps ingame? It would be a fine way to calibrate HDR settings natively, better than current logo over black or white square thing.
It would be really easy, but It wouldn't help you to estimate the peak brightness, it would just show you the raw data.
The white logo is to help you eyeball the point at which the display can't do anything else with the data.
I think the choice of logo used (I like the flashing one in The Xbox Insects Demo or a checkerboard) makes a difference as to how easy that is.
Devs should always be placing that on a full black screen too if the user is to estimate it.