Answering my own question ... I rather think that you cannot. I replaced my old C callback-set-function with this--it does exactly what I wanted in the first place: 
static SV * fp = (SV*) NULL;

void call_funky() {
    dSP;

    if(fp == (SV*) NULL)
        return;

    PUSHMARK(SP);
    perl_call_sv(fp, G_DISCARD);
}

MODULE = sometin  PACKAGE = somethin

void
funky(f)
    SV *    f
    CODE:
    if (fp == (SV*)NULL)
        fp = newSVsv(f);
    else
        SvSetSV(fp, f);

[download]

In reply to Re: XS N' void (*f) (void) by jettero
in thread XS N' void (*f) (void) by jettero

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