I wanted a Macro Keypad, but I didn't want to design a case and a plate from scratch. Instead, I bought a used Goldtouch GTC-MACB 23-key Keypad from eBay and gave it a lobotomy.
See here for an album of my pictures.
http://imgur.com/a/kstZs
I went beyond a simple lobotomy by replacing the switch matrix. But... It's definitely possible to replace the Cypress cy763813 with a Teensy. You'd just need to rewrite your TMK matrix.c routines to handle the weird matrix and live with a 3KRO.
For reference, here is the original switch matrix...
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Format:
<Chip Ref Des> pin <Pin #>
<Keycap Name> (Silkscreen Name if different) - <Switch Pin #>
Note:
U2 pin 2 is the Num Lock LED and is active low.
U2 pin 1
Clear (Num Lock) - 1
Enter - 2
Esc (ESC) - 2
U2 pin 3
- - 1
U2 pin 4
7 - 2
8 - 2
9 - 2
U2 pin 5
* - 2
Delete (BACK SPACE) - 2
U2 pin 6
1 - 2
0 - 2
. - 2
, - 2
U2 pin 7
4 - 2
5 - 2
6 - 2
U2 pin 8
Tab - 2
= (/) - 2
U2 pin 13
Delete (BACK SPACE) - 1
9 - 1
6 - 1
3 - 1
. - 1
U2 pin 14
Tab - 1
8 - 1
5 - 1
2 - 1
0 - 1
U2 pin 15
Esc (ESC) - 1
= (/) - 1
/ (*) - 1
7 - 1
4 - 1
1 - 1
+ - 1
U2 pin 16
Enter - 1
* (<--) - 1
, - 1
U2 pin 17
Fn (#NAME?) - 1
- - 2
Clear (Num Lock) - 2
U2 pin 18
Fn (#NAME?) - 2
* (<--) - 2
+ - 2
3 - 2
2 - 2