I'm with Mearls on this one.
I'd be inclined to allow any caster to use a simple melee weapon they are proficient in as their focus (whether it is a quarterstaff, a pact dagger, a holy mace, or a ritual sickle), as long as they pay some extra gold to properly modify the weapon for casting. On that same token, I'd allow any gishy caster (rangers, paladins, blade warlocks, valor bards, war/tempest clerics, etc.) to do the same with martial weapons.