Hi,
Well I will throw a couple of comments in the hat!
I have the same issue on a couple of ecom sites I monitor, internal search results are showing in the serps for various terms, and am pondering what to do about this because as you say, it doesnt seem very sustainable long term and you never know when the axe might drop.
My concern with what you suggest re redirects is two pronged:
1. If you are getting decent traffic and/or conversions from these terms currently, then you run the risk of losing these for who knows how long if the redirects for whatever reason don't sit well with google.
2. If you are redirecting what amounts to internal search queries would that not mean you would have to re engineer the search system/urls to some degree? That doesn't seem a undertaking to be taken lightly!
My thoughts might be to try a parallel approach in terms of letting the search results stay in the serps and at the same time working to boost the relevant product pages you were planning to redirect to.
If you can get those pages ranking for the same terms then if/when google decides your search results don't belong in the serps then you will have a ready made replacement to a degree. You could also try linking to the relevant pages at the top of the search results page instead of redirecting. This might help push the product pages higher and perhaps more importantly could help with your immediate problem of many visits but few conversions on these pages. That being said, wouldn't the search results page be linking to these product pages anyway?
My two cents!