So I've managed to cobble together a solution that seems to work pretty well - thanks to everyone for the help. I took Makahi's suggestion and downloaded UMPlayer, which does everything I need - one-key shortcut, no subs on caps, choose destination directory - except specify .jpg as the file type. Then I followed totoum's link to the automator workflow, which was much easier to implement than I expected, and it seems to work so far - it's certainly fast. The H x H post was capped and converted on the Mac.
Now, I still wrote and published it on my netbook, because I haven't had time to try and Mac alternatives to WLW yet. I'll give Mars Edit's free trial a shot, but based on the reviews I'm not optimistic.
What I want to do here shouldn't be hard. I need to take screen captures, without subtitles, in jpg form and save them to the directory I want (preferably with an easy keyboard shortcut). But I can't seem to find a player that can do all those things the way MPC can for Windows.
I've tried three forks of MPlayer. MplayerX takes screenshots and lets me choose the file type, but I can't find any way to take them without screenshots. Mplayer extended (both builds) doesn't seem to allow me to choose anything but PNG (way too big) or choose the destination directory, though it will take captures without the subs.
So I tried VLC, though I'm not a fan. It allows me to set a directory and choose .jpg, and even lets me choose a keyboard shortcut. But I can't figure out any way to take a cap without the subs.
Taking caps through the OS should be an option - you can choose the format and destination through terminal, and change the shortcut in System Preferences. But any caps taken by OS X have subs, no matter the player.
If anyone can offer me a way out of this box, I'd be very grateful - my OCD is not allowing me to deal with this gracefully. It shouldn't be this hard, should it?