okay, bear with me here, it's been a while since i worked on a project where i had to do this.... but i think you can constrain the ankle manipulator (or whatever control you're using for the leg ik) directly to the platform. then you can use the connection editor to link them together so that you can see the parent in the channel editor. the value should range from 0 to 1. so when the character is in the air, you can have it at 0, then when you want it planted on the platform, you select the manipulator, and the attribute will be there in the channel editor, and you can just change it's value to 1. making sure to key the beginning and end of the landing, of course. if you have any issues with the channel editor or setting the constraint, maya help should be able to walk you through it.