Unfortunately, Scribus doesn't link to text documents, you can only insert their text.
Without putting too much thought into it I can imagine linking a text frame to some text in another document would be a nightmare programatically.
For instance, how would you handle formatting? I.e. If you apply some formatting to a bit of text and the original text is changed, what do you do with the formatting? You can know exactly what changes have been made to the content but you have no idea of the intentions of the formatter. You could end up creating loads more problems by second-guessing the user.
And what about escalation? Just checking one document might be possible, technically, but if Scribus has to keep checking dozens - or even hundreds - of files for changes and then re-jigging frames to cope then that'd put a strain on the system and probably create a lag in the UI.
(I know Scribus links to images but they're easy, in comparison, to cope with changes - they're essentially just a re-load/re-display type of refresh operation.)
And I'm sure there are way more technical difficulties than the simple ones above.
And that's just for documents on the same system. For distributed docs, for example in Google Docs, the Scribus document would have to have some way of repeatedly accessing the distributed files to see if changes have been made and sending requests via different security systems. (Would the SLA have to hold the logon details? What about when you share the SLA? etc.) I don't know how this could be done but I'd say it would be outside the scope of what Scribus is about. And you still come up against the same problems I mentioned earlier.
I could be well off the mark here and someone might say that I'm totally wrong and it's easy to implement - or that I'm just a ranting fool! - but I'm fairly sure what you want isn't something that will be a up-and-coming feature in the near future.
It would be nice though!