Well, phooey. Looks like avconv in libav-tools will accept all but the -hide_banner option…so it accepts the command to do the coversion, but dies after about 30 seconds. The search results I get for that suggest that its a lack of available memory.
So…
Unless somebody knows how to limit the memory that avconv can use, it looks like this program will NOT run on a pi. It’s probably best that it not be run on a pi anyhow.
I’m going to reenable the download link, since my original ffmpeg code works just fine.