Then you'll need to modify the code and recompile. The place I saw testnet being referenced was here:
void BitcoinMiner()
{
...
while (vNodes.empty() || IsInitialBlockDownload())
{
Sleep(1000);
if (fShutdown)
return;
if (!fGenerateBitcoins)
return;
}
...
bool IsInitialBlockDownload()
{
if (pindexBest == NULL || (!fTestNet && nBestHeight < 118000))
return true;
So currently it's hard-coded to not even try to generate blocks until after it has 118000 unless it's on a test net. Among other things affected by the testnet flag are the genesis block.