Being fairly new here I don't know whether this is old news for other people here, but in case it's new and useful to anybody...
Yesterday I discovered that you absolutely can do your business standing up without requiring an STP. It works like this:
You square up to the bowl or the garden plant you're about to water, and unzip. You then use two fingers to flatten your labia minora and lift, so that the jet of liquid is directed out instead of down. And that's it! Obviously, you'll want to practice your aim before you try it in public, but that's what the shower is for. This method also has a downside in that you have to touch the anatomy in question, but I find this is offset by the convenience of being able to answer the call of nature without specialist equipment.