Guillaume-
As you are aware that, the functionality which you are expecting is not feasible. As per my understanding it is not possible to do the changes to the inventory in the previous period which is already closed.
You have to this adjustments in opened period only.
In order to do rectification to the financial values which are impacted due to closed period inventory adjustment- I can suggest as below:
1. Do the all inventory adjustments in open period
2. Create Manual inventory GLs
3. Post the difference as adjustment entry with respect to the fiscal year and period.
With this you can satisfy your Balance sheet as per the fiscal year
Regards,
Javed