HI @Nicolas_Fischer 
Marie here jumping in for @Natalia.
This update is quite new, I have just posted an announcement in the Forum as we’re ramping it up to 100% of our customers (see: Taking bulk actions in Asana List view just got easier!).
You’re completely right about not being able to edit fields that don’t appear on your project list; I’ve just gone ahead and moved this thread to our #productfeedback category to allow you and other forum members to vote for this to be improved in a future update. Our team is currently monitoring feedback and I’ll make sure to keep you posted via this thread once we have a better solution for this use case. In the meantime, if there is anything else I can help you with, please let me know!
Ps:
That was not possible before, but if you haven’t yet, I would recommend adding your vote to Create multiple tasks copies at once