Hmm... I guess this is simple enough to do with a hex file hack...
My hex files no longer resemble the originals since everything's in a different order now. But if you really really want to play with it...
There are two places where the firmware decides which layout to use.
In the 109 firmware, this occurs at locations x07DF and x0901.
In the 209 firmware, it occurs at locations x081B and x0C2D.
The instruction in question reads 30 E9 07, which is an 8051 opcode for "Jump 7 bytes if the Port 4.1 (DIP Switch #2) bit is clear". The string only occurs at the two points in question, so you can simply search for it rather than hunting for the address.
Because the Caps/Num/Scroll locks are implemented with a pin controlling their LEDs, it's actually a fairly simple matter of changing the bit you're checking.
As far as I can tell, the DIP switch pins on the TECK live at:
EA = Port 4.2 = DIP Switch #1
E9 = Port 4.1 = DIP Switch #2
B2 = Port 3.2 = DIP Switch #3
B1 = Port 3.1 = DIP Switch #4
The LED pins on the TECK live at:
B5 = Port 3.5 = Num Lock
B6 = Port 3.6 = Caps Lock
B7 = Port 3.7 = Scroll Lock
I recommend using Scroll Lock, as that won't mess up anything else. (Seriously... who uses Scroll Lock for anything these days?) As noted earlier in this thread, the NumLock keypad is hard coded into the key processing function, and disabling that would require significantly more surgery on the code.
Note that the bits in question are high when the lock in question is toggled off and low when it's on. (It's the reverse case for the DIP switches; a bit set to 1 means "on" as expected.) Also note that the "off" position of the DIP switch defaults to the Mac keyboard layout. If you want the Windows layout to be the default, change the jump instruction from 30 (JNB) to 20 (JB).
So my version reads 20 B7 07 (Jump 7 bytes if Scroll Lock is off).
Obviously, you'd have to redo checksums and stuff as usual.
I would also recommend adding Scroll Lock (47 hex) to a conveniently accessible button on both layouts, as by default it's only available via Fn-(top middle key).
Adding a third layer for NumLock is a non-trivial change that requires completely rewriting several of the key processing functions. It actually doesn't look like it would be super difficult to do, but at that point it's way beyond simple hex file hacking. Unfortunately, implementing temporary modifier access to a layer (e.g. with the Fn key or perhaps some other private internal keycode for a layer modifier) would probably be a lot of work, as it requires changing major aspects of how the keyboard operates.
WARNING: Modifying the executable portions of your firmware is a potentially brickable offence! Hack the code at your own risk!