Allowing change of name but not number

Does anybody know how to prevent users from editing a document number but still allow them to edit the name?

Number is auto generated and immutable in OIR, but this has no effect when later renaming.

Can't use ACL as name and number are both grouped under "edit identity"

Already tried the following in type manager with no effect

1. Adding immutable contraint to number attribute
2. Setting number attribute visiblity to read only

Regards

Darren