oops, updated the original post with the correction.
Yes, you can choose any enemy. The data at 21DE00-21DE2B is the 4 byte data sets. The first byte is the enemy index, and the third byte is the newly disovered lock setting. Rubble is 5F 00 03 00 as you can see. You can change the 5F to any other monster index, for example.
With a little more work, you could add additional enemies if you wanted to. Just add 4 bytes using the above format. Just make sure that the entire block is terminated with FFFF FFFF. There's some free space after that block for expansion, but I'm not sure if Geiger added that free space to TF or not. For now, you should probably leave it alone, but it's good to know that it's something that can easily be expanded and relocated later.
Also, I forgot to mention, that the lock setting only disables clicking tech and item menus. If you want the "Lock All/Can't use Techs!" textbox to appear and the blue sphere, that's in the enemy AI as far as I can tell.