There isn't actually any way heat can escape from the keyboard, cover or not.
The ventilation is by the hinge, and regardless of display angle, can vent air that way.
Unless your keyguard covers the hinge area, there isn't a problem regardless of how hot your keys get.
I would say the keyboard cover was getting hot because the area near the ports on the right hand side, and the area where the vents are (directly in front of the keyboard towards the screen) are the two hottest areas on the MBP. Since the cover is plastic it will heat up quickly.