Do you mean some concrete code? Several times I wished similar feature. To get a UTF-8 cache if it exists and encode to UTF-8 without creating a cache otherwise. 

The private _PyUnicode_UTF8() macro could help

if ((s = _PyUnicode_UTF8(str))) {
    size = _PyUnicode_UTF8_LENGTH(str);
    tmpbytes = NULL;
}
else {
    tmpbytes = _PyUnicode_AsUTF8String(str, "replace");
    s = PyBytes_AS_STRING(tmpbytes);
    size = PyBytes_GET_SIZE(tmpbytes);
}

but it is not even available outside of unicodeobject.c.

PyUnicode_BorrowUTF8() looks too complex for the public API. I am not sure that it will be easy to implement it in PyPy. It also does not cover all use cases -- sometimes you want to convert to UTF-8 but does not use any memory allocation at all (either use an existing buffer or raise an error if there is no cached UTF-8 or the string is not ASCII).