I'm all for alternative fuels, so "cars running on barley" gets my attention! (And I've been barely running, partly on barley, for years....).
If you ever want to see one of those 90mm towed-AT guns, just go to Watervliet, NY and ask at McDonald's for the big cannon in the park by the playground. It's in a public park, not on the Arsenal grounds.
Frankly, I think auto ejection would have been a must for those weapons; they had an immense firing signature and must have attracted return fire from all over the battlefield. Getting reloaded and ready to fire back quickly would have been supremely important....a Panther or a King Tiger is not something you just want to wound or piss off!