Mike,
I think you're right to consider the "extra post" arrangement shown in your mock-up photo, but while the extra 18" (or so) in clearance width this would create at a central "entrance" point is certainly better than nothing, it seems to me that 11'6" is still quite restrictive in terms of limiting the ability to turn a long vehicle toward either end, until fully within the building. Given the probability of future events where an outside vendor might be bringing in a large truckload of chairs, a portable stage, or sound equipment, etc., I wonder if the alternative of leaving out one post altogether (i.e., creating a 20' wide vehicle access) might be worthwhile in the long run. Could this be accomplished, fairly easily and without much additional expense, through use of a truss?
A wooden truss beneath the top plate (beam) might be bulky, reducing vehicle headroom significantly, and be somewhat vulnerable to the weather if not sheathed externally. However, the use of truss rods to increase beam spans or carrying capacity was certainly not uncommon historically, as you may have seen in old mill buildings, railroad roundhouses, etc.
What I have in mind would be essentially similar to a truss-rod flatcar, with a couple of conventional queen post castings spaced beneath the plate, with a two-part truss rod having a central turnbuckle. The truss rod ends would pass through the tops of the adjacent posts, with a heavy bridge washer and nut. It might be prudent to incorporate something to restrict sidewards strain at these locations when the turnbuckle is tightened, such as a short (scrap) length of heavy steel angle with one leg bolted through the plate/beam and the other leg abutting downward against each of the two posts on the "inward" side (i.e., the side facing toward the truss).
Someone among us with a degree in civil engineering may be able to refine this idea, and do the necessary calculations, but if the only investment would be a couple of, say, 12' steel rods, some scrap steel angle and bolts, and some extra car hardware that may already be in stock, it may be a relatively cheap way to gain a considerable advantage (while saving on the cost of the extra post, concrete, etc). Plus it would have a "railroad-like" visual appearance, creating a subtle architectural focal point for the "entrance."