Glad I found this topic! Sorry for dragging up an old topic but I don't see any other information on the wiki about this.
I'm looking for variable support as well. An example of an immediate want (notice I didn't say need lol) is the ability to call a macro with hotend first layer temperature and bed temperature parameters.
My Cartesian machines all will set initial hotend temp to 160° while bed heats up, then do a mesh bed leveling and finally heating up the nozzle to print temp just outside the print area and doing a purge line before it begins the print.
Yes I know that I can put this in Slic3r custom gcode for print start, but then I have to manage it for each printer 😁 I'd love to take advantage of just putting the macro call in the custom gcode section.