Thanks to all who responded. Yeah, in retrospect the heat exchange area is
woefully inadequate. I like the Saab solution that fits onto the cooolant
return line and can dump "excess" heat into the resevoir. I think that
some sort of insulation around the resevoir is needed in the vanagon to
keep any heat in (look at where it is located!). 
Another problem with my set-up is that the fluid drains back down the
line. Some sort of check valve is needed.
While I'm at it, wouldn't it be lovely to have the European Vanagon option
of headlight washers? The resevoir tank has two pumps, one for the
windshield and one for the lights.

Alistair