Hmm. I wonder if you could try the "getPageIDFromUrl" hook...
http://www.contao.org/en/hooks.html#getPageIdFromUrl
So that would fire right at the beginning when a page is attempting to load.
You could perhaps check that the page being loaded is a product detail page, and if it is, save the URL passed in (and you should be able to find the product ID from that URL too), and then you can also get and save the user session.
There might also be an Isotope-specific hook that can help, but I'm not sure if those are document yet, so they'd be harder to find. If you did a text search of the Isotope folder for something like
Code:
if (isset($GLOBALS['TL_HOOKS']['
That would help find most/all of them (that block starts just about every hook call).
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