f4a-
The Chinook I targeted were either in tide water and/or traveling fish so really rarely needed to get down more than 6 feet or so. Was able to do that easily with "normal" sink tips and full sinking lines (though during the 1970s I did have to splice together my own lines to accomplish what I wanted - not necessary with today's options.

While I don't target the kinds of suspended fish that you are talking about (easier to do so with convential gear) I suspect that you can put together a line that would get you down to where you want. I do fish ling cod with a fly in Puget Sound and the line I use is one I make using 28 feet of T14 follwed by a running line (there are stripper lines on the market that would probably do the trick as well). With that in the Sound I regularly fish depths of 30 feet or more. Depending on the currents and boat wind drift it would typically take a minute or so to reach the kinds of depths that you are talking about.

To fish suspend fish I would rely on my sinking line to get me to the depths wanted and steer away from weighted flies and any special leaders. I found that Chinook (and coho for that matter) often respond the best to a stripped fly (think fishing streamers for large trout). The drill would be make you cast, strip off another twenty feet of line allowing the whole line to sink to the desired depth (counting the line down will allow you repeat the same cast), and then stripping the fly back to the boat. With a sinking fly line and an unweighted fly you should be able to fish the fly horizonally through the fish (I like to my fly just above them). Fishing with such a presentation opens up a whole new world of potential fly patterns

Of course another option would be to fish the various "jig" type flies. In that case you might consider a seriously scaled up verison of lake chronomid fishing. It is at that point I just convert to my convential tackle.

Tight lines
Curt