InfoPath Dev

I want a user to be able to update a sharepoint list from a infopath form but not be able to manually update the list in sharepoint.

Without 3rd party applications.

I seems to me the only way is to create a interim list (list B), give the user rights to that list (list B) and then have the workflow on that list (list B) update items on List A.  Can workflow on a list (list B) update (List A) when and item is added to (List B) even thought the user does not have rights to (list A) ?

If so, does list B (the interim list) have to be on the same site?

I'm thinking of creating a sub-site that contains all the interim lists and keep them off my main site.

workflows run with elevated privellages, so even if the user doesnt have permissions to list a, the workflow will still be able to update the list.

Sharepoint Designer workflows are not capable of updations across sites, however you can write workflows using VS. that can operate across sites