Hi Milan,
So the message type actually is coming from the nms:delivery schema and not the broadlog. So unless there is a custom field in the broadlog I'm not seeing that you can write a channel value to the recipient.
Additionally, I think the biggest issue here is going to be the restriction of writing the value within the workflow versus updating after the fact. The broadlog is written to at the time of delivery preparation and analysis, there isn't really any place at this point where you could add any type of script to basically overwrite the channel and if you were to write this earlier in the workflow it would just be overridden by the delivery activity.
In this case based on the use case, I'd strongly recommend engaging Adobe Professional Services as it's likely another approach is going to be needed to meet your needs.
Regards,
Craig