This would work, the turbo guys do it with intercoolers, but I'd be more tempted to use other means of cooling first. ( your idea of using distilled water is a good idea to prevent mineral build up. )

In order to take some of the load off the A/C, make sure the heater valve shuts off water flow. Feel the heater hoses, ( careful,might be very hot ) if they are as hot as the uper rad hose, you have water flow that needs to be stopped.

On my 97 SL 320 I added a valve to the heater water supply. The stock system has a valve on the outlet but the heater core has small bleed line. This allows hot water to circulate and bring heat into the cabin. Something you want to avoid is to completely isolate the heater core as expansion of water might cause to burst. ( not likely but possible )

The added valve is a good quality plumbing type 3/8" pipe thread ball valve coupled to a 3/8" pipe 6" long cut in half. A 3/8" pipe is just over 5/8" OD making it a good fit for either 5/8 or 3/4" hose. I think it is a bit of a stretch for our metric hose but a little bit of carb cleaner spray on the hose ID does the trick. I add a heater valve to all of my cars / trucks. It makes a huge difference on the ones that don't have AC.

My SL also has / had heated windshield washer tank and heated wiper park location. While both of these have a thermostat, taking them out of the loop made for less chance of leakage.

Another thought would be to install well a guarded heater core under the bumper but in the air flow. This would help a bit but I'm not sure if it will make enough difference.

Is the A/C condenser / rad clear of leaves and bugs? Take a look between the condenser and rad for any clogs.