I have actually changed that deck, so I don't have those set of cards together, but I think the whole thing is a mess. But, for the sake of arguement, how does Riders of the Red Fork work with Maester Luwin's ability? The FAQ says that Luwin does not work with Galbart Glover because searching the top 5 cards of your deck does not count as "searching your entire deck" (even though Galbart Glover doesn't say "entire deck"). The Riders of the Red Fork have the same exact working as Galbart, so are they also not considered to "be in your deck" if you are just searching the top 5 cards?
The ruling is that Riders meet the citeria for the search effect for the entire
search effect. The reason Riders works with Rhaegal isn't because "attach it" is general (while "put that location into play" is specific); it is that the Riders, in meeting the criteria of the "search" part of the effect, also meet the criteria for the "attach" part of the effect.
So, for the "search your deck for a location" portion of the search effect, they meet the criteria of "a location," right? We can agree on that? The point is that for the "and put that location in to play" portion of the search effect, they still meet the criteria of "location."
Your comparison of Riders and Gallbart in relation to Maester Luwin is not a good one.
Gallbart says that you search your deck, you can search your discard pile instead. The ruling for how that works with Luwin is dicta, saying that if you are only searching a subset of your deck, Gallbart doesn't apply. "Search your deck" therefore does not count when searching anything less than your deck. However, the Riders say that while they are in your deck, they meet all search criteria. Well, if the Riders are in the top 5 cards of your deck, they are still "in your deck." Being part of the subset does not stop it from also being part of the general set. So the text/ability of the Riders is still in full force whether using Luwin's ability because the Riders themselves are still part of your (entire) deck, even though Luwin is only searching through part of the deck.