Well... Here is what I did if anyone is interested...
I stored the location of SSIS Packages at Runtime in some database table and used an Expression to change the value of the File Connection used in the Execute Package task.
I would have liked to get the master package current location on the fly, hence avoiding one more configuration data, but it's not a big deal.
Having said that, if you know how to do that, I am still interested! :-D