Is there any way for starting an scribus script from an external python script?.
Yes, in my own kicking around with building a Scribus custom workflow I have found ways of “pushing” content (document fragments) into a Scribus *.sla file.
The usual approach is to have Scribus GUI open and launch a python script through Scribus window > scripts to "pull" content into Scribus. Each script must import scribus.
However in some scenarios Scribus may not be present.
The approach I have tested for my own ends is to regard the template *.sla as an *.xml file and then use xquery script to update the xml elements in the *.sla document without the need to launch Scribus GUI. The *.sla templates are held in an xml nosql database. The xpaths to textframe or imageframe or even styles must be known in advance and these can easily be found from any xml editor. This is indeed “pushing” content instead of "pulling".
In your workflow you would have a daemon watching your folder and on change trigger the xquery script to push content into xml elements.
Python elementtree will also do this.