Unfortunately it's not currently possible. There were some conversations about this feature here (see use case 2 and comments). And as far as I remember Fog Creek Team were going to consider this feature, but there is still no information whether it will be implemented.
EDIT:
In general, virtual users are intended for assigning cases to a group of people. And there is a description when you create an user:
A virtual user can't log on and
doesn't use up a license. You can
create virtual users to assign a case
to a group of people. For example, you
could create an "Up For Grabs" virtual
user that owns a case until someone
assigns it to themselves.assigns it to themselves.
Also, recently Tyler has covered here new feature of Kiln 2.3 - ability to assign code reviews to virtual users.
I assume, that the main purpose and benefit of assigning permissions for virtual users is the same as for normal users - you can restrict access to specific repositories. So, you just can't make a mistake and assign case or review to a wrong user (e.g. if you have several teams and each of them uses their own "Up For Grabs" virtual user).