Scope macro, alternative syntaxes, and use cases

Several people have brought up questions about limiting the use of a 
domain name to different sets of hosts based on scopes.  For example,  
william(at)elan.net wrote:
> My understanding is that scope parameter type of macro. How can this be
> used if I want to have different list of ips (or in general different 
> set
> of records) for different identities, i.e. for ehlo valid ip are
> 192.168.1.0/24 while for mail-from its entire 192.168.0.0/16 ...
Meng replied with this example:
    mydomain.com                "v=spf1 redirect=%{e}._spf.%{d}"
    helo._spf.mydomain.com      "v=spf1 ip4:192.168.1.0/24 -all"
    mail-from._spf.mydomain.com "v=spf1 ip4:192.168.0.0/16 -all"

And, as was pointed out, this requires two TXT records to do the job.

Several alternative syntaxes have been proposed that would make such a 
thing simpler, and fit in only one record.  Indeed, we thought of some 
too.  We rejected them because, for better or worse, SPF is actually 
deployed and we are wary of changes to the syntax that will break 
existing parsers.  Success has its downsides!
Along these lines, I'm looking for a reasonable example that motivates 
such a situation.  The above example is too contrived: Really if you 
trust all of 192.168.0.0/16 to use your domain name in MAIL-FROM, then 
surely you trust any such host to use it in HELO.  Guarding against 
your own errors in configuring your own machines isn't good enough for 
my purposes.
I'm being a stickler here because people who read internet drafts are 
vultures - if they can find one tiny thing wrong they'll use it to 
dismiss the whole thing (this is also the reason why we've broken it up 
into so many drafts!)  When I say "most domains can use just one 
record, and the answer is the same for all scopes" I have to be right, 
and when I say "here is where you need records per scope" I have to 
right too.  Also, eventually, real sys. admins. will read this 
document, and the examples have to be realistic - since they will tend 
to simply cut-n-paste.
        - Mark

Mark Lentczner
http://www.ozonehouse.com/mark/
markl(_at_)glyphic(_dot_)com