I think inside a cupboard door isn't a bad idea, but for me it would create issues with the attachments bumping up against whatever is in the cupboard and perhaps the door not closing.
We have a low false ceiling over the stove area where the mixer lives, which is a possible option for a MixerMaid, but we're hoping to remove that ceiling to re-establish a ventilation system so my search for a MixerMaid location continues. I really like the concept. I just don't have a lot of options for it.