I don't see that sign even vaguely looking like an hour glass on it's side or otherwise. Looking at it again, I think it's something that Harry made with his staff and burned into the rock, we see what looks like fragments of molten rock here and there and possible tendrils of power coming from Harry's staff. Also note, and I saw this the first time I viewed the cover, that Harry's pentacle seems to be glowing. I didn't mention it the first time because I thought I was imagining it. However, the pentacle hasn't been mentioned in a long time either, nor the jewel from his mother that was added to it back in Changes.
I believe the last time we saw Harry do anything with his pentacle; other than perhaps use it for some mild lighting, is using it to walk the ways in Changes. The last time we saw it brightly glowing is when Harry used it as a weapon to kill the loup garou in Fool's Moon. (Wow, that is a long time ago.)
I think you are right about Harry using his staff to carve the symbol, whatever its purpose is. I have several thoughts about this, but right now I don't want to get into WAG territory. My main thought is a little more basic, about the nuts and bolts of magic Harry uses. I'm trying to remember the last time Harry did something new with magic, that didn't come from Winter or someone else.
In Battle Ground the banner that drew people to follow Harry was a function of Harry being the Winter Knight. The device Harry used in Peace Talks to fool Embenezar came from Molly and her magical abilities. The thing Harry did with Carlos cloak was kind of a cheap trick to distract Marcone's guests from the breakout of Thomas. Plus, we saw Harry animate some broom sticks all the way back in Storm Front so we have already seen Harry move inanimate objects around. When Harry summoned an army of little folk, that was just on a much larger scale than any summoning of Toot Toot and friends than any Harry had done before.
I stand to be corrected, but the last time we saw Harry add something to his repertoire of magic skills, that was his own, was back in Cold Days when Harry was able to use his name as a weapon to confront the Walker, HWWBf. Yes, that was a starborn thing, but it came from Harry, not someone else.
The sigil on the cover that was (apparently) created by Harry maybe something new that Harry learns to add to his magical skills and I think that would be pretty cool.