I have really gotten used to the "Simultaneous Vi Mode" functionality of KeyRemap4MacBook which lets you press and hold the s and d keys simultaneously and then use the h, j, k, and l keys as the arrow keys. When s and d are pressed together, no s or d event is fired, but when I type normally I don't notice any delay problems with hitting s or d individually.
How could I recreate this functionality in Linux (specifically RHEL 6.4)?
A couple ideas I had:
If s+d could be remapped to a modifier, then the keys h,j,k,l+modifier could be mapped accordingly. (The keyboard I am using now does not have any spare modifier keys available to use instead of s+d, plus the point of Simultaneous Vi Mode is to keep all fingers on the home row. CapsLock is already remapped to Control and Escape).
Ideally, I would find a way to do this using system settings or something like XKB keyboard which I am using already. If that's not possible, perhaps something similar to the xcape program (which I am using and which lets keys be mapped to different keypress and modifier key symbols) could be written.
xcapetriggers off of key release though, so it would have to be tweaked to keep the s and d keys from being too slow.