Remember seeing a pictures of the great John Grimek doing BB/DB movements while upside down, in a very old magazine years ago. Don't recall if he used straps (plain looped straps back than) or hooked in the hock of his knees. Funk may have something like this in his collection?
Probably a decompression board would be best for the average trainee. Getting into boots can be a challenge for a lot of people. Been told that people with high blood pressure may want to avoid this type effort. When beginning to use this position, may want to ease into it..slowly. Reason I would suggest a decompression board as first choice....set the angles you.
People who follow different forms of Yoga do headstands, and such, with good results. Seems to bring a newer supply of blood to the area, but they start out slowly. Or use NO (nitric oxide... which some guys swear by) for better blood circulation. Not endorsing this, just from what I have been told and read. I'll do 90 degree headstands a couple times a week...3 to 5 (correction) minutes.
If you want to try the upside down thing, might also suggest handstand presses. Either free style or legs supported against a wall, door, etc.. Excellent shoulder girdle exercise (delts/triceps). The great Paul Anderson included them in some of his training. Can have the hands on blocks or low raising push-up bars...etc.. all for extra depth from the bottom position.
Good Luck.
