then I fail to understand what you plan to do with the encrypted stuff that you get from the researcher since the only one able to decrypt it would be the vendor. At that point the scenario is: researcher says this is vulnerable, vendor denies it, somebody needs to decrypt the content of what the researcher has sent to the vendor. In contrast what happens without having the encrypted content is: researcher says this is vulnerable, vendor denies it, the researcher can publish the un-encrypted original advisory on your website if he feels like it.
Regardless the SSL trick is a nice one I just don't see the point of the third-party involved and I somewhat doubt that a website like this can be useful to the end-user or to put pressure on the vendor.