Is it kosher to splice wiring in the "can" portion of recessed LED lighting?
You are in code as long as there is enough room for every thing and you do not have to modify the can. The can would be considered the junction box. Also you need to make sure you have installed the correct cans for the type of insulation that is around the can, you should be okay. ie If the can is installed in an area that is surrounded by insulation the can must be rated for it, or a shroud must be built as an air gap for those that are not. 



Without the correct can, if the can has a thermal overload, the thermal overload will over heat and shut down power to the light and become a nuisance.