Would it be possible to extend the sidedef brightness feature so that the value could be set independently on all three areas of a line?
At present, sidedef brightness affects the whole side. Therefore if you have, say, a step with LIGHT3 on its face, but a regular texture on the uppser surface of the same line and want to set LIGHT3 to have a higher brightness than ambient for that sector, the upper would also be bright:

Of course, there are many more situations where this would be useful and there are editing ways around many of them but I feel this would be useful.
So, possible? Is it within UDMF specs?