Don't know if my writing will be descriptive enough but here's a try. I bought the Selway, similar to the Pintler, to work as a big-ish day pack, small-ish 1 or 2 night pack. I planned on hooking a big hip sack to the top for a lid when out for a few nights. I have an ancient Dana Design hip sack that fits the bill.
If you look at the Selway pictures on the MR web site, you'll see the webbing that goes over the top of the pack, starting at the top of the load lifter attachment to the frame and ending at the top of the daisy chain on the back. The Pintler pics don't have this webbing.
Short answer: I used this webbing to attach a hip sack as a lid to my Selway. I bet you can call MR and get the same straps for a fee and jury-rig a system that will work with a hip sack, dry bag, very small duffle, or, maybe best yet, a sleeping bag stuff sack with compression straps.
Longer answer: I took some split-bar/repair buckles and threaded females into the load lifter connect on the frame, males into the top of the daisy chain webbing on the back of the pack. I then made a strap system that threads through various straps on my Dana Design hip sack, male ends connecting to the females at the top of the load lifter, females to the top of the daisy chain webbing. Works like a charm!
**Disclaimer: The reference to male and female buckles connecting to each other is in no way intended to cast judgement or create ill will toward those prefer male-to-male or female-to-female connections. These references are only made toward the use of webbing and straps.
**Also. I don't have any packs or frames to compare my Selway to, haven't put on a Stone Glacier, Kuiu, new Sitka, Exo, Kifaru...Nothing. However, I really like this pack. Carries excellently and synches tight with a light load, expands to carry a big load.