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My 1997 (for parts) and 2000 (needs those parts) Ford Explorers both have OEM heater core bypass valves in them, vacuum actuated. Not just a block-off - they have all 4 hoses hooked up to the valve, it's a bypass.
They also have a blend door, that is apparently a failure-prone part that's not fun to fix, being well-buried in the dash. Haven't studied the system any closer than that.
A heater core with hot water in it doesn't do de-humidification. Hot air has a lower RH, sure, but still has the same mass of water in it. You need the A/C evaporator core with evaporating refrigerant in it (or ice-water circulating through your heater core) to de-humidify by causing water to condense out on the "colder than dew point" surface it presents, and allowing it to drip off and run out the drain pipe under the car (or on your carpet, if that pipe plugs up).
Incidentally, this is why the windshield defogger position on the HVAC dial in a car generally has the A/C system turned on - you want dry air blowing on the back of the windshield. Hot dry air is faster than cool/cold dry air, but wet air sometimes just fogs it up worse.
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