Isn't a lot of this going to depend on the specific pack fabric? Should we assume you have something basic like uncoated nyon or polyester?
I have a Seek Outside Unaweep in XPAC, and I went through and seem sealed all the stitching (inside and out). As the base fabric is pretty much waterproof, and the stitching should no longer wick, I should now be pretty dang close to waterproof (the critical things like my sleeping system and puffy are still in dry bags).
My daughter has an Osprey, and we just accepted that the nylon was just going to get wet and suck up water. We put a trash compactor bag inside, and folded the opening down the back. That should be plenty, unless the entire pack is submerged (again, dry sacks within). It's also only takes 2 seconds to fold the liner back down into the pack. We trimmed the trash compactor down though, as she only has a 19L pack.
Back in the day, I was prepping for a longer AT section hike that never happened. Pack covers were strongly discouraged by most all the through-hiking community in most of the research I saw. Basically, if you encounter sustained rain somewhere like the AT, the pack is just going to get wet, and you can't stop it. Pack covers are heavier than the pack liners, and leave a huge hole where the backpack attaches to the backpacker, and that entire area is going to soak through, unless you bring your entire backpack under your rain protection, like a massive poncho. Even then, you're likely going to need to take the pack off at some point.
