i have the first button at the top left, since that is naturally where i want to reach for it. like this:
123
456
as for reaching over the bottom row, why does everyone always make a big deal about that...it's not really much of a reach and it's not like they get in the way. i guess maybe if you like to press your hand flat against the panel, then that could be an issue, but when i put my hand down on the panel, i don't press my entire palm down on it, so my fingers are naturally curved and the bottom row sits right below the part of my hand that isn't touching the panel.
as for remapping unused buttons, that is a great idea. i was just rewiring my panel last night and testing out a few things and i though about doing that, but i wasn't sure it if was worth the effort. people sometimes ask which buttons to use, but usually you tell them once and they are good. (or the press the buttons that don't do anything and then move on to the ones that do)