Yeah, it's usually the latter - and when we do know it's Adobe RGB we do exactly what you recommend, going via CIELAB - but it's not precise, and clipping does

Yeah, it's usually the latter - and when we do know it's Adobe RGB we do exactly what you recommend, going via CIELAB - but it's not precise, and clipping does occur. We're leaning towards "please upload a valid image" as a solution, which is a shame, but the whole perceptual end of things makes doing this automatically nigh on impossible - can't come up with a solution that works for all cases.