I've used 3 different ISO files:
Kubuntu-Active-13.04-desktop-i386
Kubuntu-Active-12.10-desktop-i386
plasma-active-2 (from balsam)

I've used 2 different USB sticks:
Kingston DataTraveler 2GB
OCZ Rally2 4GB

I've disabled Secure Boot and fast boot, and successfully booted normal Kubuntu 13.04

I've tried making the USB the primary boot in the UEFI bios, and I've tried choosing the USB in the boot menu (pressing F10 on power on)

In all cases Plasma Active doesn't boot and after a 1 second delay Windows 8 boots instead.

Should I post the same message to the mailing list? Should I post a bug report? Should I ask in the IRC channel?

In this case it seems that your USB devices have not been converted into bootable media successfully.
I'd suggest checking if those ISO images are suitable for use with USB drives, and if they are, verifying the process you are using to write them to the device.

The ISO images specifically say to mount them on a USB stick (tablets don't even have disc drives). I was using Unetbootin to mount them, but just to make sure I started up my linux box and used USB-Creator. That didn't work either.

Hmm. Unusual. Have you checked to make sure that secure boot is disabled? I'm not sure if the Plasma Active images are are able to boot on system which have secure-boot enabled.

In my original post I stated I disabled Secure Boot and I've gotten regular Kubuntu to boot (which I've now installed for the time being).

Is there a way of installing Plasma Active on top of a normal Kubuntu install? There are applications in the repository that seem like they are plasma active, but I can't figure out which would be required.

If I recall, Plasma Active for Kubuntu was available at one point - you had to add some PPA repositories however, and manually install some packages. I'm not sure however why it is not booting properly - you may want to try http://en.opensuse.org/SDB:Live_USB_sti ... structions instead possibly.