Well you need to check. With my experience on several customer, we never experience any locks.
But it would be good to test it before going with "real time" update, if you get such locking...
You must keep in mind, that CMOD is an archive, and normally the indexes are also part of the archiving document.
Some countries, like Switzerland, don't allow changes in the metadata without some business approval and some compliant report showing the change, etc...
The only exception is some "status field". where they are not part of the document itself, but more from a workflow perspective.
That's why the application group fields are per default "not updateable", because this is against the goal of an archive to modify the data.
I am just saying that, without knowing what you are planning to do. But I felt the need to tell you this important matter... because you might in your case be impacted, or not!!!