Note: ext4 (and other linux file systems) are mounted when honoring the files permissions, so one can't probably write to the root of the disk but *can* write in any folder belonging to him (same UID)

USB disk

 udisksctl mount --block-device /dev/sdaX --options ro

I even can use /dev/disk/by-label/xxxx as it will probably always be the same source (to various targets)

loop iso image

 udisksctl loop-setup -r -f openSUSE-Leap-42.1-DVD-x86_64.iso

Option "-r" is to mount read only, omit it to have rw.

then

 udisksctl mount  -b /dev/loop0p1

to umount:

Notice that udiskctl use "unmount" and not "umount"

 udisksctl unmount  -b /dev/loop0p1

 udisksctl loop-delete -b /dev/loop0

source: https://wiki.debian.org/ManipulatingISOs#Loopmount_an_ISO_Without_Administrative_Privileges