\mount -o loop filename.iso /mnt/mountpoint
Mounting an ISO as read-write ∞
There is no way to mount an ISO as read-write. Although it's annoying to implement, it really should be implemented, as it would solve so many problems.
One trick around this is to mount an ISO, selectively copy it out to a HDD for changes and then turn that tree into an ISO. Pain in the ass, but it works.
This doesn't handle issues with making ISOs which conform to certain filesystems though. One would have to have a partition set aside which has the filesystem of preference on it, to have the directory tree there.