It may be easiest to setup a samba share and then in the
tilemill config file point the files dir at the samba share and
then use samba to determine who has what permissions. Otherwise,
upstart will run the tilemill process as the mapbox user and you'll
want your files dir and files to be read/write by mapbox. Samba may
just make this easier to deal with.
For whatever reason this AMI hasn't been able to start, so I
can't debug any more (or check to see if I changed that path, who
knows what I was doing?). I'll close this for now hoping it was
something dumb I was doing.