per-user data signatures [was: Re: multiple keys vs multiple identities]

[Phil Brooke]

On Fri, 24 Sep 2010, David Shaw wrote:
> There is actually a defined field for this in OpenPGP (see section 
> 5.2.3.22, Signer's User ID).  I don't think anyone implements it though.

Is there any particular difficulty or reason for it not being implemented 
by anyone?  (It looks very similar to, for example, the policy URL 
signature subpacket.)

Thanks,

Phil.