Just wanted to check back in and say thanks to all for the advice. My inclination was to take the car over to Hill Country Euro (based on X5chemist's recommendation and also because they're on my side of town), but in three days of calling them, I didn't get a callback or a person answering the phone one single time! Who knows, maybe they were on vacation or something.

Running out of time, I decided to give the AC Pro product a try. The gauge showed my car on the low side of the pressure range and I just added a bit until I was in the middle. Seems to be working fine for now.

I suspect I've got a slow leak in one of the recharge valves, so I expect that repair is in order very soon.

If anyone sees a Black E53 on the road between Austin and Chicago tomorrow, it'll probably be me. And if you see the windows down, you'll know my AC fix didn't work. :-)

Thanks again,
Chris