I'm in the process of modding my UK layout AT model F. So far I've added keys at each end of the space bar and put in some extra hammers. Just the space bar stablizer bits still to do... and then of course remappings.
So I thought I'd list here the scancodes generated by each extra pad, as read by Windows using a native PS/2 port:
code - USB code - description - location
0x57 - 44 - F11 - lower pad under keypad '+'
0x70 - 88 - Int'l 2 (Katakana/Hiragana) - right hand pad under Left Shift
0x71 - n/a - VK_OEM_RESET - left hand pad under Space
0x72 - n/a - unassigned - right hand pad under Space
0x73 - 87 - Int'l 1 (Ro) - left hand pad under Right Shift
0x74 - n/a - unassigned - left hand pad under Enter
0x75 - n/a - unassigned - upper pad under Enter
0x7C - n/a - do not use - left hand pad under Insert
The USB codes and descriptions are (mostly*) taken from a Microsoft document called "USB HID to PS/2 Scan Code Translation Table". For those keys that have USB codes, some PS/2 to USB converters use them, others ignore them. All the converters I have (except for the blue cube) ignore the keys that have no USB code. The blue cube helpfully remaps 0x75 to 0x2B (the code for the key left of backspace), so that's perfect if you're only planning to do the big backspace mod as per the article.
Going by that, I expect that no standard PS/2 to USB converter will pass all the codes through, so I plan to make my own...
(* VK_OEM_RESET is a virtual key definition taken from a Windows header file)