Having found that MCE remotes cost ~£15 and an Arduino micro + IR receiver + Remote can be obtained for much less (or free in my case as I had all the parts laying around). I decided to make this sketch that emulates a USB keyboard to control Plex when it receives button presses from the remote. It could be easily adapted for other applications too.
The IR codes in the sketch are from a NOW TV (Roku) remote. The sketch will print out any unknown IR codes it receives over the serial port so if you wish to program in your own remote simply press each button on your remote and copy down the values printed out.
The default Arduino USB stack does not support waking a sleeping PC. To support this, I have used NicoHood's HID library. Instructions on how to install it can be found here
Along side this you will need to install the IRremote library
You may get an error message about IRremote.h existing in another location. This is usually benign as it will use the one you have installed but please check the error message to verify this.
I have also included an eventghost script that I use to allow me to switch between Plex Home Theatre and Steam. I also used InstantSheller to start eventghost instead of explorer, allowing to me to boot straight into PHT and have the ability to switch to steam via my remote.