Thanks for all the responses. I wasn't meaning to complain about my tripod - I'm quite pleased with it overall, and would recommend it to other people looking for a CF tripod. It's just a specific use that I hadn't thought much about, given that I didn't own a macro lens until a couple weeks ago.
You guys all had some good points that I hadn't thought about (especially about how I might want the intact column later, even if I don't use it much now). I can always buy a replacement, but they're not cheap. I will say my concerns (when upside down) aren't about stability - they're about whether I trust the head+plate to hold with a 2 pound camera and a 2 pound lens hanging from them.
I realize it's probably silly that I'm uncomfortable using the reversed column - after all, they wouldn't design something that'd drop my camera, right? But I just couldn't relax with my camera hanging there, upside down...
The other issue (that may just be due to unfamiliarity with using it that way) is that, upside down, the legs tend to get in the way a bit. The 70-180 macro is a fairly large lens, and setting everything up so the lens isn't against a tripod leg means the viewfinder of my camera is next to a tripod leg. It's a bit awkward.
Abstract, I had been thinking about trying a different Manfrotto short column just to see if it works. But then someone else just pointed me towards this
low angle adapter. I didn't know this even existed - I'm going to give it a shot. If I'm not happy with it, I'll try shooting "upside down" a bit to see if I get more comfortable with it - I might have to keep a pillow around to place underneath, though, just in case.