I just got back for a week. I had one day at AK as an offsite guest and the rest of my stay as an "onsite guest".
At AK I went straight to a FP+ kiosk and waited about 10 minutes,(about an hour after opening) but then waited about 15 more minutes until I was walking away for the reason that follows. They were very confused because I was onsite the following day but offsite the day I was there. I don't know if that was the reason but they couldn't book my FP+ through the online system. So after a long wait they eventually just gave me a card that had my 3 FP+ selections loaded on it. It was a card for that day only.
We didn't use our fastpass+ at Kilimanjaro since the standby was empty, so we thought we would go to a kiosk later in the day to pick another ride based on wait times. When I went back (don't remember time) I asked if we could switch out Kilimanjaro to Dinosaur. They tried, but the system is set up that you have to change the TIME on the ride you have first and then change the attraction, we were out of luck. This is because Kilimanjaro was out of FP+ for the day. So even though there were Dinosaur FP+ we couldn't access them. THAT to me is a major problem that I hope they will change, or maybe we got a CM that didn't know what they were talking about...
ANOTHER HINT: (Hopefully they will continue to do this.) As we were walking from our car and were heading to the monorail to MK this past Saturday, at about 8:45 AM we were approached by a CM with a ipad and she was offering to sign us up for FP+ selections right there. (we were far to the right and only saw it that one time, but maybe scan the area and see if you can avoid the lines inside)
I learned, as others have stated, it is imperative even more than ever with FP+, to get there for RD if you tour commando style like we do. If I had to do MK as an offsite guest I would still go hit the big rides first and then go get in the FP+ kiosk line. I know people have posted some long lines and I'm sure there have been, but there were also many times I say FP+ kiosk lines that hardly no one was in, and some machines even open.
Overall we didn't get to re-ride the rides a ton of times like we use to, but I have prepared my kids before hand that that would be the case. But we still were able to hit RD and hit every ride we wanted for Epcot and AK in one day, Studios in 2 days, and MK in three days. We weren't planning on MK for three days but with only 3 FP+ each day we decided to do it three instead of the two days we usually do. (3 days was to REALLY get our fill and then some

)
Hopefully this helps with someones planning purposes, taking into account this is low season.