Is it wrong to write HIGH to a pin used as an open collector?
It's not really wrong to set the pin HIGH, but doing so may cause output contention. That is, you may get into a situation where your code is driving the pin HIGH, but the device at the other end is trying to drive the same pin LOW. The PS/2 mouse (and keyboard) protocol uses open-collector signalling to avoid contention. In an open-collector system, signals are assumed to float HIGH when they are not driven by a pin, i.e. when the pin(s) are high impedance. A pin can safely be pulled LOW by the device at either end of the wire.

So, it's probably best to use TRIS in this case.