Name translation problem using kerberized NFSv3 with Active Directory

[varun mittal]

We are trying to setup NFSv3 Kerberos with Active Directory. The server is
configured with Active Directory using 'winbind'
Our NFS server recognizes the AD user names in the format
'domain\<username>' [due to winbind in nsswitch]. The name format is fixed
and cannot be changed (some policy constraints).

Problem:
The issue we are having is that the principal name in NFS RPC layer is of
the format 'username at realm' . This leads to idmapping failure when NFS
server tries to map the name part of the principal name to a UID [since
'username' doesn't comply with the format ''domain\username']

Probable solution:
There is an option in krb5.conf 'auth_to_local' which provides name
translation which can be solution my problem

Currently the RULE format in auth_to_local supports the form [from file
lib/krb5/os/an_to_ln.c]:
                "s/" <regexp> "/" <text> "/" ["g"]

What I need is something like this [the way they handle in sed]:
                "s/" <regexp> "/" <regexp> "/" ["g"]

so that I can 'append' the 'domain\' to each of the incoming 'username'

Queries:
a) Is my understanding of the above potential solution correct ? Or does
there exist an alternate solution to this issue ?
b) Is such a mechanism already available or is in pipeline ?

Note: I cannot use the DB:<filename> since number of users is too high and
change frequently.

Thanks in advance