Hi Laure,
I don't think there is any easy to use mechanism for this in Galaxy.
Unfortunately Galaxy loads all the *.loc files at startup and treats
them as global values available to all users equally.
I can't find the email thread, but I recall someone previously trying
to do something with a single large example.loc file for all users
but with an extra column for filtering by account name. I forget if
they got these changes to work or not.
Assuming you only have a few separate versions of the example.loc
file, you could create copies of the BLAST wrapper XML file which
use the specific *.loc file - and then restrict the tool by user?
In either case at a minimum you would have to maintain local
changes to the BLAST wrappers (and potentially changes to
Galaxy itself), which is not a good situation.
Peter