Yep you’re right, when I’ve reported the issue I’ve forced to remove my Silverblue installation so I haven’t found time until this week to give a new try to Silverblue.
This morning I’ve reinstalled Silverblue with 2 layouts installed (default us + fr-oss) with the hope to use the special keys to switch between them also on the boot. The switch key works in Anaconda and inside Fedora but not on the dm-crypt boot.
Here is the results of cat /etc/vconsole.conf :
$ cat /etc/vconsole.conf
KEYMAP=“fr-oss”
FONT=“eurlatgr”
$ sudo rpm-ostree initramfs --enable
[sudo] Mot de passe de guillaume :
Checking out tree c77995e… done
Enabled rpm-md repositories: fedora updates
Updating metadata for ‘fedora’… done
rpm-md repo ‘fedora’; generated: 2019-04-25T23:49:41Z
Updating metadata for ‘updates’… done
rpm-md repo ‘updates’; generated: 2019-05-23T17:37:23Z
Importing rpm-md… done
Resolving dependencies… done
Checking out packages… done
Running pre scripts… done
Running post scripts… done
Writing rpmdb… done
Generating initramfs… done
Writing OSTree commit… done
Staging deployment… done
Freed: 1,6 Go (pkgcache branches: 0)
Initramfs regeneration is now: enabled
I rebooting… results here in few seconds 
EDIT: Still in QWERTY for the boot password 