Retested again tonight ... and it's now working !
It's really strange, because yesterday, I could not get any file to be created by DOpus in the /dopusdata/Script Snippet ... I even tried to put some (js files, expecting the file name to be associated to the snippet name).
And now, when I create a snippet, JScript.js.xml is created or updated.
I did 2 things in between : unplugged USB key I exported DOpus 12 before switching to beta, and fully restarting DOpus (systray icon + exit, then launch).
Anyway, everything works as expected ... except maybe the inability to use snippets in script buttons which could have been nice 
but I guess there might be a reason behind it, I guess I will live with it.