Thanks Josh,
In the main we rank very well for medium length & long tail searches about our plants, which made me confident about doing all this internal linking in the first place. It wasn't until the die was cast that I lost my nerve.
It's so hard to track rankings across so many products & keyphrase variations, especially when we add in the variables of ongoing link building & the fresh chunks of content that we added to almost all of the updated product pages when we put in these new internal links. We're tracking samples of the most valuable ones in SEOmoz campaigns and nothing dramatic has changed.
We have no accurate means of measuring whether or not these links have affected support calls & email traffic, which isn't much help! We knew this beforehand, but our reasoning was basically "how could it hurt?".
I suppose what I'm really asking for is an example of another site that has dealt with this particular issue, i.e. found a way to use (almost) site-wide links to necessary but commercially unimportant pages without draining the link juice of its important pages.