It's difficult (if even possible) to have macros "hold down" the button like that. My advice - hold the power button, and then use a macro to setup the rest of your system.
If you're handy with hex codes and have a pronto available, you may be able to create a superlong IR code that you can teach to the MX-500, but that's going to require quite a bit of trial and error!
David
www.BlueDo.com