ono I am not really convinced by your 'compelling reason'
you already have flux hanging out of emission which you have written is actually watts x luminous efficiency
I could do a wee wangle there in the UI and present flux as efficacy scale input and multiple the 2 parameters together under the skin to give flux...
I have thought about this useablility and what I would like to do is to keep my UI the way I have it for emission being an extra dimension for a material (ie simple glow - no IES access there), but also keep 'meshlights' (or simply 'lights') in the mats menu (actually as a mat under the skin but with reversed emphasis.)
ie meshlights will appear not unlike present with IES but there will be a simple menu at the bottom to choose a generic material type for reflectivity of the light source/mesh
so in other words they will actually reference the same things but in a different order/way.
seems to me there are many cases where the light source doesnt need to be thought of as a material or with all the trappings and options of one.
I guess its just a matter of logical/intuitive presentation to the user
if you convert the present meshlights stuff internally is it not possible to keep the parameters for that hanging out of the code for us to retain 'peak' , 'blackbody' etc?
of course other script writers may have other ideas and seeing as how I am on the outside of things here ordinarily my opinion is not so important... still I'll persist with this enquiry in the meantime
You could definitely present a UI to the user that has a luminous efficacy box, plus a power drawn (wattage) box, if you want. Then, as you said, just multiply the numbers to get the luminous flux. Also you could have a default luminous efficacy of 13 lm/W or whatever.