The 9mm axles typically on front hubs may be a little more of a problem than ball size. But a longer solid 10mm axle should be possible to use for this fixed setup. I've done that on two front disk hubs, a Deore and a Formula. I think on the last one I used a 185mm long 10mm leaving plenty of room for spacers and outside nuts. The idea of bolting cogs directly to a disk mount is attractive. Drilled cogs in various sizes are available for less than $20 and are probably easier to change than a thread-on cog with locking ring. Another advantage is the option to set up a double cog (twingle) using small thick washers on the screws as spacers between the cogs.