My dropout is 6mm steel. Is there enough meat on a 5mm MKS to be ground out to fit?

nope – you should use the MKS fat tugs instead which are 8mm wide

out of stock at hubjub though

But I want a sleek Allen type…

Surly ones look nice and dont have enclosed clasps so will work on fat frames.

SO do On-One ones but they have 2x allen bolts with small locking nuts. They dont look very purty though.

You could grind the ends down on the skinny ones, but it’d be tricky unless you just take the whole shoulder off so they don’t come round the sides.

they are very hard metal. have a set of fat ones that were not quite fat enough to fit my old klein dropouts, and despite a lot of effort i could only remove about 0.0001mm of material.

You are welcome to my MKS chaintugs for £10 posted if you want?