Actually there's no reason why earth-leakage protection could not be put on the secondary side of these tool transformers. The center-tap earth arrangement doesn't in any way preclude RCD use (the RCD would need to be one designed for 110V of course).
The RCD works on the principle of comparing the current in two conductors and tripping if they differ by more than a set amount. There's no need for one of those conductors to be earthed (a neutral).
If you wired something like an American GFI (Ground Fault Interrupter) on the secondary side across the two hot/live lines you'd then have a 110V supply, with no more than 55V to earth, and with earth-leakage protection that would trip on about 6mA.
