> (Per the current SRFI, you first need to try to write an object, and if
> that fails, you get back an exception satisfying "unwritable-error?")

Sorry, that's a misleading claim on my part. The SRFI says:

> (unwritable-error? object) => boolean
>
> Possible causes for the error include the following.
>
> * The port's syntax cannot encode any unreadable data at all.
> * The implementation does not know how to encode a stand-in for this particular object.
> * The programmer has set an implementation-defined flag saying that trying to write unreadable data to the port should raise an error.

But the SRFI does not provide a way to set the flag in question.