whoops my mistake but if bmx companies supply their bikes with 10mm dropouts then surely you could search for a bmx hub with a 10mm axle?
The majority of quality rear hubs are 14mm axles.
On the topic, I've done it to a couple frames and never had one break. Just make sure to mark everything out beforehand and check that you have enough material above and below the newly cut opening.
I would do it for sure. I would use a tapered 14mm drill bit which would centre itself in the slot and then grind in from the ends to meet the hole. I am not sure if they make this for bikes but my sons stroller (Off road stroller with qr wheels) has axles that have flat spots on opposite sides which reduce it allowing it to slide into smaller dropouts, seems to work good?
do it ! seriously 2mm either side isnt really anything at all dude and to just make sure you aint gonna brake anything have your rear wheel slammed so that it doesnt have any leverage on braking the mount which it is unlikely to do