The last post I read about this stated that you couldn't get Heatran early in Pokemon Diamond and Pearl. Well, this may work. For this, unfortunately, you need 6 gym badges instead of two. I DO NOT HAVE EXACT NUMBERS FOR THIS. (Well, now I do, just check the edit) Do this on an emulator so you don't lose all your save data. If someone figures out the right numbers and posts them, I may put them up on this post, if I can edit it. Since this is my first post on the forums, I'm still a little fuzzy on how things work. Anyways, it's the exact same as the 15-minute Darkrai glitch by Aera, until closer to the end. After you SR after getting to the battle tower, and hit the invisible wall directly east, and then the one directly north, here's what you need to do. Just like in the Darkrai glitch, go 9 steps south, and start biking east. Now, keep on going until you get under Stark Mountain on the berry/marker map on the Poketch. Go north until you hit the invisible wall. Keep going 1W until you are no longer under stark mountain. Then, head north until your location is shown as being right next to Stark Mountain. Go east. If you hit an invisible wall, you may have to go north or south until you are no longer against it. When you find an opening, go east for a little while. If you go through Stark Mountain, you've gone too far. Here's where I haven't tested it out enough to know how many steps you need. At some point when you're shown as being in Stark Mountain, save and SR. If you got lucky and didn't stand in a wall, you should be on Stark Mountain. On my first try, I appeared on some stairs that were pretty close to the entrance. Unfortunately, I don't have the sixth gym badge for strength, but I will update this when I do. Anyways, I'm working on a crappy video showing it, but that's not finished yet. See ya.
EDIT: Okay! I found the number of steps you have to take! When you are directly under Stark Mountain, go up and you'll hit an invisible wall. Repeat 1W until you are no longer under it. Repeat 1N until you're just to the left of Stark Mountain. Then, go 12E and 20N. Save and SR. You should be right before the entrance to Stark Mountain's insides.