Sorry, but repeating much of this reply in several threads - a lot of veneering questions popping up all at once! I recently took a semester-long veneering class -
http://www.palomar.edu/woodworking - where we each made numerous projects including starbursts, mariner's stars and game boards. Be glad to cull my class notes and handouts and send more detailed replies to specific questions. We talked about big projects, but no one did anything on the scale you're talking about. That'll be one big a$$ bag!
My personal experience is that joewoodworker.com is a great site for info and supplies and he has excellent customer service. There's tips there for making your own bag. If this is a one shot project, you can get by with much thinner, lesser quality bag material. I have some class notes and articles I could send you re: mechanical presses, vacuum presses and bags.
Vacuum press?. . .
If you make a bag that doesn't leak, you wouldn't necessarily need a huge pump. Once the initial air is withdrawn (which does take longer with a small pump) it shouldn't be that difficult to keep the vacuum constant. But it's gonna be a PITA to get your piece inside a bag that big, keep the veneer lined up, get the forms/cauls in place, etc. You'll probably need a positive and negative form for a curved surface. I've seen vacuum bags snap MDF forms like popsicle sticks when they're not supported. You could end up with a 72"
flat panel.
Mechanical press? . . .
Seems like it'd be an easier setup. You could consider pressing it in sections rather than having to deal with putting on a 3' x 6' piece of veneer, lining it up on a wet surface, etc. Maybe like when veneering a column, you wrap the veneer around the column but don't glue up the overlap part. After initial part sets, come back, cut excess to make an invisible joint, then glue up that part.
PM me if you want more info.
Mike