I try to point out things that I wonder about first of all...
But, the fact that you have 4:3 screens when everything on the market is moving towards widescreen is of a real concern to me. You have 12'x9' screens, which were awesome back in the days of 4:3 computers, and 4:3 TVs, but at this point, they are significantly outdated.
So, if you want to stick with those screens, I would still get a widescreen projector and keep the 12' width, but forget about filling it top to bottom.
That said, all you need now is math.
A 12' 16:9 screen is 81" tall. About 7'. That's about a 84 square foot screen. Call it 80 square feet.
Realistically, in a conference room setting with 'normal' fluorescent ambient lighting, you want 80 ADVERTISED lumens per square foot to achieve a minimum 10:1 contrast ratio on screen.
80 times 80 is 6,400 which is the lumen count you are looking for as an absolute minimum at that width.
With a 9'x12' projector - you are at 108 square feet (4:3 aspect ratio) and at 80 lumens per square foot, you are at about 8,600 lumens as a minimum projector requirement.
Sticking with widescreen, this is the list I would be looking at...http://www.projectorcentral.com/projectors.cfm?g=1&hide=0&st=1&mfg=&p=&w=&r=&br=6500&br=15000&ll=<
All of these have interchangeable lenses. I did not put a maximum on price, but they get crazy expensive. It is sorted by price, so the least expensive model is about $5,500.
Only TWO of these are called out as 'short throw' projectors. That's just reality. A good image doesn't really happen with short throw projectors, the optics are crazy expensive, and it just isn't a widely required solution with large screens. The reason people most often buy short throw projectors for business use is for interactive white boards - and those images are far smaller than your request, so you really aren't going to get much in the way of short throw, but a short throw lens is often available - about 10'2" is the ONLY spot that Panasonic can throw a 12' wide image from with their .8:1 fixed short throw lens.