I see there are hardware thin clients available. But what about a software client than can be run on another linux PC, does one exist?
My goal is to have 1 linuxMCE box and would like the ability to watch TV on another existing linux PC.
I'm not doing what you're attempting, but I've gleaned some approaches from the forums and the wiki.
The existing PC can be set to PXE boot across the network as a MD, which would do what you want. Though it would of course not do anything the existing Linux install did, as that OS instance wouldn't be booted. However, you could mount that MD's local drive by using its orbiter to go to the Kubuntu Desktop, mounting, and then running old installed apps from that drive, as long as they're compatible with the LMCE Kubuntu install. You can also install other dependencies into the LMCE OS to match what's on the old Linux install, but they'll disappear whenever you reboot the MD. Unless you upgrade the MD image itself that the MD machine boots from the Core. ddamron has posted some notes of his own research into that subsystem in the Devlopers forum, and perhaps someone will update the PXE boot
wiki article with more details.
FWIW, you can config your MD PC to PXE boot from the Core, but then config the Core to direct the MD to boot from the MD's local drive instead of from the Core's PXE image for the MD. Which means you'd only config your MD machine's BIOS once to always PXE boot, and then reconfig the Core's switch of "PXE/local" to determine what your "MD" PC runs as when it's done rebooting. A UI to that switch in the MD itself would be a good LMCE feature, like a "Reboot A/B(/C...)" switch that changed the Core's pointer to whichever image (PXE/local/etc) is required, with each image option given a meaningful name (eg. "Bedroom MD" or "Bedroom workstation" or "Livingroom MD" etc).