Of course, you will never be able to reflect a color state in a CT channel, so the CT channel should not be used for rule input in case of color lamps. So we only have to keep the HSB value accurate.
As an alternative way of keeping an accurate state, the binding could use the native CT channel, but additionally update the HSB state with an approximate value for the color temperature. So basically, do it the other way around. I found with the philips lamps that hue 45 and saturation 0-60% does the trick.
That way, the lamp shows the color temperature the way it’s maker intended, and people could still build rules that react to HSB updates exclusively. Even if in that case the HSB state wouldn’t exactly be true to what the lamp shows, for rules it would still be good enough. You could even “reserve” a certain hue, e.g. 45, to indicate that it is actually in color temperature mode. If anything commands 45 by HSB, send out a 44.