IIRC ULA had a plan to run an internal combustion engine off the hydrogen/oxygen boil off, providing power for stage systems and or cooling/recondenser in their

IIRC ULA had a plan to run an internal combustion engine off the hydrogen/oxygen boil off, providing power for stage systems and or cooling/recondenser in their ACCESS upper stage concept. Of course ideally you don't want to loose any propelants to boil off but it still seems like a nice idea.