We need to be careful here.
That said, another possibility would be to change it to:
SEQUENCE OF AttributeDescriptionOrSpecial
AttributeDescriptionOrSpecial ::= LDAPString
where the LDAPString is contains a value conforming to the production:
attributeDescriptionOrSpecial = attributeDescription / specials
specials = "*"
and a statement:
The specials production may be updated by standard track
technical specifications updating this document to allow
additional specials. Servers MUST treat specials they do
not recognize as unrecognized attribute types.
This leaves the exact form of a future specials up to
future standard track technical specifications.
If one wanted to allow non-standard track specification of
specials, then one could write a standard track specification
defining a family of specials that have disambiguated by
an IANA registry of specials. I rather leave this to others
(e.g., not LDAPBIS).
Kurt