[racket] scribble and delayed pre-parts?

Is there a mechanism in Scribble for defining a delayed part?  I think
I'm looking for the equivalent of the traverse-element and
traverse-block constructs, but at the level of pre-parts.  I'd like to
conditionally generate the content of a Scribble document at decode
time.  I'm staring at the data definition for pre-parts, but I don't
see a structure analogous to traverse-element or traverse-block.  I
want to make the decision at decode time rather than compile time to
avoid some funny issues that might happen with regards to 'raco make'
and bytecode from scribble documents.