You could "technically" ride a Hardrock with a 100mm RST fork on some DH runs. It just will not be fun at all. Since the Hardrock is built as an entry level bike, it will not do well on a DH trail. A longer travel fork won't do much to help you: Yes, it will help the fatigue and stress on your arms/wrists/hands, but the rear end will still be bouncing around everywhere.
Quick question: Have you ever ridden your Hardrock with someone that has a DH/FR and switched bikes for a rock garden? I actually just did yesterday and we hit some gnarly rock gardens. Now, while I was able to clean them on my hardtail, when I jumped on his rig, I was quickly reminded of why a full squish bike is SOOOO much more fun!
If you absolutely can not get a full squish bike, and you want to make your bike more "DH worthy," the best advice I can give is get some Maxxis Minions!
Sorry for the long-winded response, but I hope this helps.