Fragments of 2030 Question

Nope only when its on the field.

Unless the Craft Essence itself specifies that works with the backline equipped servants, it will only work when that specific servant is on the frontline.

For example, event CEs from Onigashima event that gave +50% attack boost, worked with backline servants and that meant even if they died in battle, you were getting the same boost from them for the entire party for the duration of the battle.

This also means that most Bond level 10 craft essences that give a party buff won't work if they're equipped to someone else other than the specific servant whose CE that is and if they're in the backline.