I haven't used LinuxMCE yet (though I have used Pluto Home).. Pluto had excellent support for X10 devices (
www.x10.com)
X10 has a device called "The Big Red Button" which should work great for firing off a pluto event. They are cheap too - the button, a tranceiver unit and chime come to under $20.
I have had excellent results with X10 products with Pluto - they aren't as great as ZWave (there is a 2 second delay in the tranmitting with X10 products) but they do work great.