You can overboost in reverse thrust, I have seen it happen.
From 31 Oct 2004 FCTM, page 6.38:
"After touchdown, with the thrust levers at idle, rapidly raise the reverse thrust levers up and aft to the interlock position, then to number 2 reverse thrust detent. Conditions permitting, limit reverse thrust to the number 2 detent. The PM should monitor engine operating limits and call out any operational limits being approached or exceeded, any thrust reverser failure, or any other abnormalities."
The logic is probably that Boeing are giving you the option to use all available thrust to prevent an overrun say for an RTO, aquaplaning, flapless or generally poor landing.
S&L