You can add «Custom action» to mount ISO file from within Thunar file manager. ¹⁾ This only method I know to mount UDF ISO files from GUI.

To add a context menu action in Thunar, open the Thunar file manager anywhere click on Edit → Configure custom actions: ²⁾

• Name: Mount image
• Description: Mount an ISO disk image
• Command: ³⁾ udisksctl loop-setup -r -f %f
• Under «Appearance conditions» tab - File Pattern: *.iso;*.ISO
• Tick 'Other files'

Thats it! Now you can mount an ISO image file in Thunar via right mouse click.