I have not heard of such a thing. Usually Marketo would push the value to the SFDC record if SFDC was blank. Who told you otherwise?
SFDC leads that get pushed to Marketo in this situation will NOT be deduped, so you may end up with lots of dupes and not actually overwrite any of your existing records.
You could also do the initial Connection, which then allows you to review the mappings before initiating the sync. That should allow you to display Field Blocking.
We tested this out in the sandboxes. If we cleared the data from a field in SFDC, it also cleared in Marketo during the sync.
My understanding is that the first sync is unidirectional with SFDC being the master, while subsequent syncing is bidirectional. Is this incorrect?
I also don't see how to do an initial connection without sync. The CRM page in the Admin asks for SFDC credentials and there is only a "Sync Fields" button.
I see now that the leads coming in from SFDC are going to create duplicates in Marketo rather than overwriting existing data. So this isn't a problem, although the duplicates are going to be a problem.