This works, as long as the Fn keys are layer-lock keys. If you use Keymap_MomentaryNext (the only kind of “next” layer key we have at the moment), it needs to be present on the target layer too, otherwise when releasing it, the firmware would not know it was a layer key. (Or… hm… I’ll have to verify this… it may not be the case anymore!)
In any case, using MomentaryNext needs quite a bit of care. A perhaps safer approach would be to use the MagicCombo plugin.