Water wetter is very good stuff and even helps in "normal" glycol based systems. I run it in all my heavy-duty applications.

There is a new glycol product that is supposed to be used without water as it has even better heat transfer properties even if it cannot hold as much specific heat in it's mass (must pump more volume/unit time).

Increasing the cooling system pressure will also raise the boiling point of the coolant a few degrees but it can be hard on the system, especially heater cores and water pump seals.

Rusty water can't be good! Especially considering that thin tube from the pump to the heater. I'd be less concerned with plugging than with the obvious question, "what is corroding so much?"

Mike!