Hopefully you get more current information as
HHN heats up.
I can only tell you what we experienced last year with the HHN cup. There were some online sources that said you had to have a Frequent Fear pass (I think that's the one) to purchase the cup; other sources didn't mention it. I think almost every 'official' source also said the refills would not be available during regular park hours, but plenty of reports had come in that people were able to refill them all day.
We did not have FF, we were using BOGO passes. We tried to buy at three different places (at the kiosk in front of Monster's Cafe, in Monster's Cafe, and at Mel's). All three asked to see our FF passes.
At the kiosk, we said we didn't have that pass and the TM said they couldn't sell to us unless we had them but said she probably would anyway - except she was out of stock. lol Inside the cafe, they said they thought we needed the FF pass to buy (but were not sure at all) and weren't willing to sell them to us.
At Mel's, the employee said they weren't supposed to sell it unless we had FF but they would ask a manager. Two managers came over and had a brief mumbling conversation then eventually said they felt we wouldn't get our monies worth (since we only had 2 HHN nights) but added, "If you want to buy them, we're fine with it".
Since we knew they also worked during the day, we felt they were going to be worth it - and they were to us.
We bought the HHN cup during regular park hours and used it both during regular hours and at HHN but there were a few "only if..." statements that worked in our favor. If they clamped down on either of those, the cups would not have been worth it for us.