The problem may not be the class of ?| per se, but the interaction with ?: .

The whole thing |rdf|:|someClass| should be considered as a symbol.

At the moment, using smartparen to push a ?) through this symbol,

you start by putting it here :

|rdf|):|someClass|

then here

|rdf| :)|someParen|

(notice that a space has been added)

and last here 

|rdf| : |someClass|)

Smartparen relies on syntax tables for motion and categorizing symbols.

Its author pointed me to the syntax class of ?|, and I actually wonder in CL

if ?| shouldn't be part of the symbol rather than a string delimiter.

In CL standard syntax, it is also used in multi lines comments

#| ... |#

There maybe other uses I don't remember.

Fabrice