There are two good reasons for not using HCA with RC paper:
RC paper is designed for minimum wet time. After a while the liquid starts to soak into the base through the edges, at which time the "advantage" is gone.
It doesn't need it: Since the base doesn't absorb anything during the intended quick processing, there is no fixer to wash out of the base. The emulsion alone can be washed in a relatively short time - long before the water starts soaking in.
So in the interest of keeping wet times down, HCA is not recommended. And if you DO use HCA, you'll need a longer wash to get the HCA out of the (now soggy) RC base.