If you are worried about the condensor blocking the radiator then you could do
what they did with the condensor on the '70s buses, put it under the van with
two fans on it to shed the heat.  I don't know how well this setup worked but
I have seen it on a couple of vans and it looks neat and could be easily done
on a diesel Vanagon.
I really think the reason VW didn't want you to put a A/C system on the diesel
is that the motor is already under a strain just hauling the van, and its
occupants around.  It shows by the head problems that these usually
bulletproof motors had in the Vanagon application.
I love diesels and when I first got into Vangons I considered a diesel.  I
didn't care that it was slow but when I found out that the heads went bad
every 100k or less I decided that if I wanted head problems I would rather
have a waterboxer to deal with and get 10 less mpg but get a good bit more HP.
To me the diesel Vangons failed because they didn't have the two things that
other diesel VW have.  Excellent fuel mileage (my Jetta gets 50 mpg and I
think 30 is not good enough to justify the low hp) and longevity (my parents
Jetta has 250 k miles on it and just stopped burning oil!).
Thats my 2 cents.
Ken Wilford
Van-Again
John 3:16