I formated a usb pen with one partition (type tested : ntfs/exfat/hpfs fat32, fat32lba, 0x7....) with mkfs.udf (see udftools) the partition type seems to be unrelevant (https://serverfault.com/questions/35243/what-is-the-partition-id-filesystem-type-for-udf : Short answer: I would suggest to use MBR id 0x07 for UDF partitions.) and just selected to allow windows to detect the udf disk windows detect the udf disk fine linux find the partition (say sdx1 for example) #mount -t udf /dev/sdx1 tmp works but thunar don't list the device into the available mount options….
I cannot reproduce here, I have just formatted a 4gb pen drive with gparted and then cfdisk + mkfs.udf, in both cases it works fine, the device appears on side pane as LinuxUDF. Which version of Thunar are you using?
closing as "works for me", since there is no further info from reporter which could help to reproduce the bug