If someone else feels like tackling this, please go ahead, and let us know what you come up with :)

Your demo script works for me as is, if you change the XSub (my_print) to:

void my_print(SV * fh) {
    dSP;

    ENTER;
    SAVETMPS;

    PUSHMARK(SP);
    XPUSHs(sv_2mortal(fh));
    XPUSHs(sv_2mortal(newSVpv("TEXT", 0)));
    PUTBACK;

    call_pv("MyFH::PRINT", G_DISCARD);

    FREETMPS;
    LEAVE;
}
[download]

Update: Changed PUSHs(fh) to XPUSHs(sv_2mortal(fh)). (Still sems to do the required task.)

This can quite possibly be improved (there are a few things that I ought to check regarding sins that are perhaps being committed) ... but, given the convoluted nature of what it does, I'm not even sure that it's helpful.

Cheers,
Rob