You are correct that ring numbers must be entered in hex on the 3920/3900. When configuring vlans on the Cat5000, ring numbers can be entered in hex by using the "ring" keyword, or in decimal using the "decring" keyword.
On router platforms using IOS, including the Cat5000 RSM, ring numbers are always configured in decimal form.
You will need to convert from hex to deciamal to make sure that ring numbers are consistant.