Yes it is open right at RD right after the welcome show they drop the ropes and I walk straight through the castle and PP is always my first attraction.
It is not always open.
If closed, just take the path around the left side of the castle. We did this on a recent trip, and it worked great.
We watched the Welcome show near the entrance to Liberty Square and queued once the show ended. However, we went the long way that passes the Hall of Presidents and Haunted Mansion and still managed to arrive around 30th in line with maybe a couple minute wait. There is also that narrow walkway to the right before the Hall of Presidents that bypasses the castle on the left side which may be better, but I noted people were walking slower and taking pictures along that pathway, so we avoided it.
Either way you walk will have little to no wait if you casually walk to PPF at rope drop as everyone is headed to SDMR. Line does build quick past a few minutes at rope drop on PPF though so try not to stop to take pictures or make any detours.
You need the @Robo map.

Remember this is one of those attractions where the queue is a great as the ride. So being there first isn't necessarily an advantage particularly if it means you have to rush through the queue.

We did the green Robo route through the castle (both castle entrance paths were open) in Dec. to Pan. But we made sure to ask CM first (we were the 1st to wait there) if castle route to Pan was going to be opened after the opening show or if we needed to take the blue/orange Robo routes. We walked on and fortunate (for second guessing purposes) that the 1x we didn't RD SDMT it was broke down anyhow.
I've taken Robo's orange path with success. What I find is that if I cut to the right immediately after passing the HoP and go through the marketplace area, I can move more quickly and move up in the group. The CMs at the front of the pack don't always like that, but it is what it is.
The path through the castle is shortest, but risky. If it opens even a minute later, you're going to be a lot farther back in line at PP.
I know it all depends on crowds but what is the line usually like if you'd do 7D at rope drop then head over? This would be on an EMH morning the end of august. We may not even decide on doing these that morning (will hopefully have FP for 7D a couple times before and PP isn't really a must ride but we would like to once) but am curious.
A heads-up on the blue path: we were there two weeks ago, and there were CM's positioned on the path who were only allowing people with BOG breakfast reservations through. (This was at 8:00am for an 8:00am regular -- not EMH -- opening.) Apparently, they were giving out some kind of paper "ticket" to people with breakfast reservations, and you had to show that to get through. I don't know if it's a new procedure, something they were testing, or something they were only doing during busy periods. But, we saw people get turned back two different days.
A heads-up on the blue path: we were there two weeks ago, and there were CM's positioned on the path who were only allowing people with BOG breakfast reservations through. (This was at 8:00am for an 8:00am regular -- not EMH -- opening.) Apparently, they were giving out some kind of paper "ticket" to people with breakfast reservations, and you had to show that to get through. I don't know if it's a new procedure, something they were testing, or something they were only doing during busy periods. But, we saw people get turned back two different days.
Just taking that on face value...
I'm going to guess that it was case of inexperienced CMs unaware that their duty to restrict
non-BOG guests from that path should have ended at 8AM on that morning.
Odd opening time, coupled with isolated CMs, likely "just following orders."
Orders that would should have been better explained on an 8AM opening time.