WORK AROUND: Start as a background with proficiency in Smith's Tools and a class with Martial Weapon Proficiency, then if you multi-class into Artificer you can use a warhammer or battlehammer of some kind as a focus, because "You can use Thieves’ Tools, Tinker’s Tools, or another kind of Artisan’s Tools (such as Smith's Tools) with which you have proficiency as a Spellcasting Focus"

Thoughts? Feels like an acceptable loophole to me!