It's unfortunate they can't have a meal system like AA, or pmUA for that matter. On the convoluted mentality that special meals cost too much so you get chicken or chicken at UA, this OP question is exactly what you can do on AA.
If the normal veg choice looks good, pre order it. If not, phone in the special meal.
One thing I like about the US version of AA, is they consistently have a veg option for lunch and dinner that you can pre order/or have a good chance of being served if upgraded last minute...unlike UA where the aforementioned meat product only service, often of the same type, is all too common up front.
Per above, a real airline in International F will have a purser who presents your special and the F veg option and often opines that you may prefer off the menu. Fairly standard on LH (I usually opt for my AVML but have been convinced to order off menu a few times). This used to be the case on pmUA as well, but I can see how the 1-2 magical veg options, if that, would not be offered lest some GS/1k on an upgrade has to suffer through factory farm meat products while paid flyers are told they can buy a snack box.
The current version of UA definitely NOT veg flyer friendly