In fact, how about the following breakdown:

- One SRFI specifying `write-unreadable-object` without saying anything
normative about what syntax to use, other than that it must be something
that is unambiguous to the implementation's reader. SRFI 243 could be
repurposed for this.

- Another SRFI about the #?datum syntax specifically.

- To experiment with Scheme Review, I'll submit an explanation of this
mess there, pieced together from the current text of SRFI 243 and its
mailing list. No normative parts, just a survey and recommendations.