I think your wish is self-contradicting. `#fff` is so-called _device_ colour -- a device like a LED-based display uses it directly to drive the LEDs, where `#fff` means that the red, the green and the blue channel are already "cranked to 11". The `f` here is equivalent to 11. HDR uses a different color format, I think -- exactly because `#fff` is otherwise either ambigous, or has to map to a different colour gamut -- where, for instance, `#fff` actually means the whitest white cranked up to 11, at however many nits (say 1500) the monitor may emit, which would make your "standard" or "SDR" white (per sRGB, say) that's usually has the emitted strength of around 100 nits, be somewhere at `#888` (I haven't taken into account the _curve_ implied here, at any rate I don't think it's going to be a linear relationship between nits and device primary N-bit colour numbers).

Also, `#fff` is ambigous -- if you mean device colour, then there's no brightness (nits) specified at all, it may be 200 or 2000 or 10,000. If sRGB is implied, as in `#fff in sRGB colour space` then the standard specifies 80 nits, so when you say you don't want brighter than that, then you can't have much of HDR since sRGB precludes HDR by definition (can't go brighter than 80 nits for the "whitepoint" aka white).

I think if you want HDR you need a different colour space entirely, which either has a different peak brightness, or one where the brightness is specified additionally to e.g. R, G and B primaries. But here my HDR knowledge is weak -- perhaps someone else may chime in. I just find colour science fascinating, sorry to go on a tangent here.