I'd like to exclude certain pages from the Varnish cache based on the content of the page (for instance if the Form uses a particular hidden field which is a security feature and needs to be unique on every page refresh).
I have dozens of forms, so I don't want to have to exclude each unique page individually from the cache.
Is this possible within the VCL?
No, normally not. The proper way to do it would be to set cache-headers (for instance "Cache-Control: no-cache, must-revalidate") on your pages with the non-cacheable forms that varnish in turn will read.
As a nice side effect that will also cancel most client side caches that also often can cause troubles with CAPTCHAs and the like.