It turns out that Adobe Air provides a pretty straightforward method for converting swfs (like Hippo Gram) into Android App apk files - you can even bundle the Air software in with the apk so no one has to download anything separate! It's quite nice. Converting it from a straight swf with the Flex compiler was just a question of creating an Air project descriptor, compiling with Air rather than Flex libraries, and adding some ActionScript to force a landscape orientation. The Adobe documentation was very helpful.
Just for kicks, I've uploaded Hippo Gram to Google Play Store as a free download (and no, there aren't any ads either, this is all just for hobby fun). You can download the APK here.