The incantations for this were gleaned from all over the net and also avconv manpages. This being so, and with many non-obvious problems along the way, I figured sharing it here might be helpful to the world at large.
avconv -i - -codec:v libx264 -s vga -preset ultrafast -f mpegts -codec:a libmp3lame -ac 2 -y OUTFILE
So now, an explanation:
-i -
means 'input from stdin'-codec:v libx264
means use the libx264 codec for video output-s vga
means make vga-sized output (in a bid to lower computation cost)-preset ultrafast
is an argument to the libx264 codec to tell it to be fast rather than accurate-f mpegts
means output format of mpegts-codec:a libmp3lame
means transcode to mp3 output-ac 2
means use the first two channels and drop the rest because x264 doesn't handle >2 audio channels-y
means overwriteAfter all that, it seems my ancient little NAS is too slow a machine to do realtime A/V transcoding. Ah well, at least I know *how* now - new hardware is a different issue. And maybe this will help someone!