I suppose you could do this overriding the MembershipUser returned by the Active Directory provider, something like this:

public class MyActiveDirectoryMembershipProvider : ActiveDirectoryMembershipProvider
{
    public override MembershipUser GetUser(object providerUserKey, bool userIsOnline)
    {
        ActiveDirectoryMembershipUser user = (ActiveDirectoryMembershipUser)base.GetUser(providerUserKey, userIsOnline);
        if (user == null)
            return null;

        return new MyActiveDirectoryMembershipUser(user);
    }

    public override MembershipUserCollection FindUsersByName(string usernameToMatch, int pageIndex, int pageSize, out int totalRecords)
    {
        MembershipUserCollection newColl = new MembershipUserCollection();
        foreach (ActiveDirectoryMembershipUser user in base.FindUsersByName(usernameToMatch, pageIndex, pageSize, out totalRecords))
        {
            newColl.Add(new MyActiveDirectoryMembershipUser(user));
        }
        return newColl;
    }

    // TODO: check other methods to override
}

public class MyActiveDirectoryMembershipUser : ActiveDirectoryMembershipUser
{
    private string _userName;

    public override string UserName
    {
        get
        {
            return _userName;
        }
    }

    public MyActiveDirectoryMembershipUser(ActiveDirectoryMembershipUser user)
    {
        // TODO: do your decoding stuff here
        _userName = MyDecode(user.Email);
    }
}

NOTE: you will need to ensure all methods that return a user are overriden. It also has a some performance impact on collection methods, because you'll need to duplicate the collection (as I have shown in the sample).