Is 7bit restriction to signature practical?

Some sons of multipart/signed(aka RFC1847) including PGP/MIME and
S/MIME restricts an object to be signed to 7bit. From the UA
programmer's point of view, I wonder if this restriction is practical.

Let me give you an example.

In Europa, we can find many 8bit messages as follows:

        Subject:
        From:
        To:
        MIME-Version: 1.0
        Content-Type: text/plain; charset=iso-8859-1
        Content-Transfer-Encoding: 8bit
        
        ISO-8859-1 text comes here.

When I try to forward this message with my signature, how should I do?
I mean that I want to create the following message.

        Subject:
        From:
        To:
        MIME-Version: 1.0
        Content-Type: Multipart/Mixed; boundary=foo

        --foo

        An explanation on the following forwarded message.

        --foo
        Content-Type: Multipart/Signed; boundary=bar; 
                protocol="yyy"; micalg=zzz
        Content-Transfer-Encoding: xxx

        --bar
        Content-Type: Message/Rfc822
        Content-Transfer-Encoding: xxx  

        Subject:
        From:
        To:
        MIME-Version: 1.0
        Content-Type: text/plain; charset=iso-8859-1
        Content-Transfer-Encoding: 8bit
        
        ISO-8859-1 text comes here.

        --bar
        Content-Type: yyy

        Signature comes here.

        --bar--
        
        --foo--

Since 7bit restriction is required, I think the forwarded message must
be first analyzed with MIME syntax then be converted to 7bit.

Is this practical? How about forwarding a more complex MIME message
which is in 8bit?

I guess many implementations just include messages when forwarded.

--Kazu