I use a USB mini hard drive as the RPI's swap space and would really like to store all video and incoming gcodes onto it rather than the SD card. Is this supported by octoprint?
Yes, see https://discourse.octoprint.org/t/where-do-the-g-code-and-video-files-go/3857/5 and just change the default locations via the OctoPrint web interface.
Just make sure that you keep the drive plugged in. If the mini hard drive doesn't have it's own power, you should check the power required to run it and compensate if necessary. That 2.5A power adapter is pretty much just for the Raspi itself.