It wouldn't help. The lightning bolt is, what, a mile or more long? It hits your head, if your bike conducts no electricity it will travel through your body and out your feet to the ground. A few more inches of air travel at the end won't bother a mile long lightning bolt. Your aluminum bike is not connected to the ground, the lightning is going to have to jump an air gap at the end either way. By the time the current flows from your head to the bike frame it has passed through and stopped your heart. Sitting on a backpack isn't going to help either. Getting down low is no certain protection, I once saw a lightning bolt hit a low building right next to a 100 foot plus commercial radio tower built of nice conductive aluminum. You cannot predict what lightning will do out in the open. Aircraft traveling hundreds of miles per hour get hit by lightning, speed is no protection.

Part of the reason that lightning strikes are rare is that sensible people do not stay outdoors in lightning storms! Ignore that advice and you do increase your odds of being hit by quite a bit. I don't know by how much and your odds of being hit are still small. My lightning drill consists of not going out in lightning storms and getting indoors as soon as I can if one catches me unawares. Stop and get inside a public building if you can. Beat it for home, work, a friend's house if you can. If you are stuck way out in the country with no possible shelter then just ride. Your odds are good enough that if there is no real shelter available you might as well just continue on. A tree is no shelter and its height actually puts you at more risk, don't bother stopping under a tree.

If you can feel the tingling sensation from an impending strike you have little, if any, time to do anything. If you can I would get as low as possible and if you are off the bike try to stand/squat on one leg or with both feet very close together. If you are lucky the lightning will only strike nearby and when it does the enormous current from the bolt will flow radially outward from the strike point. If you are standing/squatting on two legs in that zone where the current is high some of it can flow up one leg, through your body, and down the other depending on how your feet are placed with respect to the current flow. Keeping your feet very close together or supporting yourself on only one can prevent that and save your life. Not that you are likely to accomplish any of this in the time you have....

Ken