RegLoadKey()(MSDN here ) を確認してください。次のようなことができるはずです。
using System.Runtime.InteropServices;
using Microsoft.Win32; 
namespace ConsoleApplication1
{
    class Program
    {
    [DllImport("advapi32.dll")]
    public static extern int RegLoadKey(uint hKey, string lpSubKey, string lpFile);
    [DllImport("advapi32.dll")]
    public static extern int RegUnLoadKey(uint hKey, string lpSubKey);
    [DllImport("advapi32.dll")]
    public static extern int OpenProcessToken(int ProcessHandle, int DesiredAccess, ref int tokenhandle);
    [DllImport("kernel32.dll")]
    public static extern int GetCurrentProcess();
    [DllImport("advapi32.dll")]
    public static extern int AdjustTokenPrivileges(int tokenhandle, int disableprivs, [MarshalAs(UnmanagedType.Struct)]ref TOKEN_PRIVILEGES Newstate, int bufferlength, int PreivousState, int Returnlength);
    [DllImport("advapi32.dll")]
    public static extern int LookupPrivilegeValue(string lpsystemname, string lpname, [MarshalAs(UnmanagedType.Struct)] ref LUID lpLuid);
    [StructLayout(LayoutKind.Sequential)]
    public struct LUID
    {
        public int LowPart;
        public int HighPart;
    }
    [StructLayout(LayoutKind.Sequential)]
    public struct TOKEN_PRIVILEGES
    {
        public LUID Luid;
        public int Attributes;
        public int PrivilegeCount;
    }
    static void Main(string[] args)
    {
        int TOKEN_ADJUST_PRIVILEGES = 0x00000020;
        int SE_PRIVILEGE_ENABLED = 0x00000002;
        int TOKEN_QUERY = 0x00000008;
        int token = 0;
        int retval = 0;
        uint HKU = 0x80000003;
        string SE_BACKUP_NAME = "SeBackupPrivilege";
        string SE_RESTORE_NAME = "SeRestorePrivilege";
        string tmpHive = "offlineSystemHive";
        string offlineHive = "E:\\Windows\\system32\\config\\SYSTEM";
        LUID RestoreLuid = new LUID();
        LUID BackupLuid = new LUID();
        TOKEN_PRIVILEGES TP = new TOKEN_PRIVILEGES();
        TOKEN_PRIVILEGES TP2 = new TOKEN_PRIVILEGES();
        retval = OpenProcessToken(GetCurrentProcess(), TOKEN_ADJUST_PRIVILEGES | TOKEN_QUERY, ref token);
        retval = LookupPrivilegeValue(null, SE_RESTORE_NAME, ref RestoreLuid);
        retval = LookupPrivilegeValue(null, SE_BACKUP_NAME, ref BackupLuid);
        TP.PrivilegeCount = 1;
        TP.Attributes = SE_PRIVILEGE_ENABLED;
        TP.Luid = RestoreLuid;
        TP2.PrivilegeCount = 1;
        TP2.Attributes = SE_PRIVILEGE_ENABLED;
        TP2.Luid = BackupLuid;
        retval = AdjustTokenPrivileges(token, 0, ref TP, 1024, 0, 0);
        retval = AdjustTokenPrivileges(token, 0, ref TP2, 1024, 0, 0);
        int rtnVal = RegLoadKey(HKU, tmpHive, offlineHive);
        Console.WriteLine(rtnVal); //should be 0
        RegistryKey baseKey = Registry.Users.OpenSubKey("offlineSystemHive\\ControlSet001\\Control\\ComputerName\\ComputerName");
        Console.WriteLine(baseKey.GetValue("ComputerName"));
        baseKey.Close();
        rtnVal = RegUnLoadKey(HKU, tmpHive);
        Console.WriteLine(rtnVal); //should be 0
    }
}
}