Ok, so I finally went ahead and ordered a 122-key keyboard from Unicomp. I thought they would be able to put a horizontal enter key, but they said no (it would be an engineering change). So I will find out when it comes in tomorrow if it has the pad above the Enter key (where a backslash/pipe key would normally be) hooked up; if it does I will have to bolt-mod it so that I can move the hammer (pivot plate) and put a horizontal Enter on it. That is assuming I can't get used to the vertical Enter. Will see.
But the main thing I was considering is what to do with the extra buttons. I have ordered a Teensy so that I can convert from PS/2 to USB, and am planning on remapping some of the keys at the same time. First, would be to put the editing keys (pgup/pgdown/home/end, etc) in the normal order. Then I was going to turn the key in the middle of the cursor keys (labeled "Home") into a macro key. The idea is when you hold down the key for a few seconds, it would activate macro programming mode. Then hit one of the extended Fn keys, followed by a sequence of keys you want it to produce, followed by the macro key again. In order to provide feedback on what mode the macro functionality is in (normal, programming, on/off, etc) I was going to add a tri-color LED under the key to provide a feedback glow around it.
Other than the macro functionality, I was wondering what else I should try programming in the Teensy? Alternative keyboard layouts? Maybe adding a mechanical relay so that I can add even more audible click noise whenever pressing a key? Or something like a pager vibrator motor to give that authentic Selectric movement when typing? Also, what other functions should I map the additional keys to? (I'm using Linux as my main OS). There are several Compiz effects I could map, and possibly application shortcuts (browser, terminal, email, etc). Maybe even add in an LCD display window giving access to built in keyboard apps (calculator, phone dialer, etc). Any suggestions welcome, even if is a suggestion that I'm a bit crazy.