I think you would be better off permanently activating the clutch by some means. Maybe take it out and blow a hair dryer on the bimetallic spring element and see what it actually does, then mechanically make it do that all the time.
The fans should be less in demand at higher speeds of the car as the air being forced through the radiator at higher speeds is probably being held up by the fan, since it only runs to 3300 rpm (see my other post on the viscous fan operation). So, if you ran the car at higher engine speeds and got over much over 3300 rpm on the fan, it might decide to come apart.
I think faking the mechanism out to think it is hot is better and safer than mechanically locking the clutch mechanism. By the way, what were the symptoms of the first viscous clutch failure and did they go away when the clutch was replaced? Good luck, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)