I should probably be a bit more clear
When the machine boots, and I hold down the option key, it will fail - straight to the full disk encryption password dialogue box. If at this point I restart the Mac, the option key will work the second time and allow me to select e.g. my Boot Camp Windows partition.
My theory is that both of these events occur somewhere before OS X is loaded, and that the BlueTooth keyboard and mouse are visible by the smart boot / openboot system. Some of this must be true because I can use the keyboard and mouse within the smart boot menu to select which boot option to use.
What seems to be happening is that the BlueTooth pairing event is taking longer than the 2 nanoseconds that the system waits for keyboard input before trying to load the default operating system.
What I'm trying to work out is whether the boot can be delayed in some way to allow for the BlueTooth pairing to occur at the boot menu level and the option key to be pressed. We know that this does occur the second time around, so it's simply a matter of time, and would make the whole boot system that much less painful.
In the meantime, I'm going to much around with something called Boot Champ, which seems a bit of a hack but hey, if it works, right?
You're probably right but as you state it doesn't work from a Cold Boot. So either use the internal keyboard or do as you have been and Re-Boot after getting the encryption password screen.
You can also select the Win partition from Startup Disk in Sys Prefs when you know you are going to boot into Windows at next startup.