Hi Digirew,
Unfortunately my knowledge on writing code (besides some tiny thing with html) is equal to zero. But I've checked the links from your comments, and the Scripter API seems a powerfull tool for python-empowered folks. It even wakes my curiosity on learning some code!
Actually what I'm doing is very simple and can be achieved just by opening a new document with the new margins and the automatic text frames option clicked, but it would be more elegant if woudn't have to start a new document each time.
There is only one column.
As you can see in this first picture, the text does not exactly start either at the top nor at the bottom (red lines).

By working around with the baseline grid offset and lowering the margins (increasing the top and bottom of the text frame), it is possible to get very close to the real/visual margins which at first I designed:

But then, after changing the margins, the new automatic text frame on the following pages wont readjust to the new margins. The solution is to open a new document and set the final margins there...
But it would be nicer to do it within the same document. Besides, there may be cases where a document would be wished to have automatic text frames with different sizes (of course you can do that manually, but damm, there are already too much things which are to be done manually on Scribus!).
If automatic text frames would automatically readjust following the master pages, that would be awesome, wouldn't it?