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.
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).