Check out this post on Blog by Florian Courgey, by our own Community MVP @Florian_Courgey, about sending Message Center real-time emails from workflows using JavaScript . This post explains how from a workflow in Classic instance A, you can send a SOAP message to Message Center instance B.
I will be very cautious to make such use cases due to Campaign constraints on the Message Center side and also on Marketing side when invoking external API, which RT SOAP API is for the marketing instance point of view.
Before embarking in such a deployment, you got a few questions to ask yourself:
What is the use case(s)?
How often will you execute such a workflow?
What is the expected volume for each workflow execution?
How do you get the data in the Marketing instance, file or API?
All these are important before pushing the solution further to build s scalable and maintenable Campaign solution.
This blog post is actually @Florian_Courgey's work so, tagging them here for an open discussion on what was their thought behind this use case and its scalability.