1) scale a colour by a float - can scale two colours or two floats, but if I have a diffuse colour map and a diffuse strength float I can't seem to combine them.
2) get a float from a colour or colour from a float - in the standard shadermixer bricks don't these automagically convert by using the average going to float and greyscale going to colour? I guess this would fix my first point too.
Standard shader mixer bricks are able to do this conversion under the hood because they are actually writing code for the final shader. The Luxrender doesn't have a shader language (well it has c++ compiled into the program but that's not quite a good experience for you to have to recompile Luxrender in order to use a custom Eluxir material). So the operations that are available are more limited. Turning a float into a color can be done with a simple mix texture, make the color inputs black and white and hand in the float as a mix value.
By the way Luxrender doesn't actual work with the same rgb"colors" as your standard biased renderers, it works with spectrum. During read it turns user supplied colors into these and at the end it bakes these spectrum into colors for your display/image, but they are a bit of a different animal. There's not a Luxrender supplied conversion from spectrum to float that I've seen and you'd probably not want to use it if it did as its a lot of computation to get a value, when just supplying a value would be more efficient.
I believe there are convertor for float and color(spectrum) into fresnel textures. These should be in the brickyard.
I agree though that bricks that took one/three floats to define a color would really useful. Especially because of the above mentioned issues.