This can be done without any special integrations on Hive.
Use a link to the form hosted off the blockchain Or build an interface around the form. Not a blockchain thing, but can be (ie. dpoll).
This can also be done, off-chain. Would require attaining the email addresses, aside from maybe esteem sign-ups (which does not store user emails on chain) not many users are comfortable linking their email addresses directly to a cryptocurrency wallet/account.
A better approach and possibly more helpful email function for the community, since we are not all email marketers and most would simply get annoyed: optional email notifications would work just fine and could possibly achieve what you're reaching for by notifying members of your posts to a specific community.
Cheers
Thanks. In regards to 1., I was thinking about the actual opt in form being in the post, as opposed to a link to another website. And for 2., I thought something like an opt in form in the right sidebar, where users could enter their email address directly and sign up for updates, as an added feature, not by linking with a wallet or a Hive email address.