I tried to a diagram of what my setup looks like...
The problem I have, even when I have a strong seal around the compressed air hose, is that there's no chance for the drain line to get pressurized: it just blows out the end that drips into the condensate pan. And I just now realized that the same problem would apply with a shop-vac.
Unfortunately, there's no access to either end of the drain pipe. The insides of the AC unit are sealed so I can't access the section that ends at the condensate pan, and the other end drains into the bathroom sink drain line.