The inspiration of the treadmill-hand walk was an exercise my high school PE teacher used, the wheelbarrow. Get in a push-up position, have someone grab your feet, and then walk on your hands with them supporting your feet. Exact same exercise, but it requires a partner.
Any dumbell exercise or series of exercises can be turned into a muscular endurance workout. Try this shoulder circuit:
Do each exercise for 1′ without rest
Rest 1′ between each exercise
Lateral raises (standing flies)
Bent over row
Reverse bent over flies
Rest 3-5′ and do it again. Start the first week with this and the second week add on another circuit.
This is just one off-the-top idea, you can design loads of other ME circuits with as few as two and as many as 10 (beyond that it might get silly) exercises.
For further inspiration check out pages 166-167 of Training for the New Alpinism.