Mike,
I think the distinctions between steelhead fishing upstream or downstream of the Dalles bridge are mostly in people's heads. Whether a steelhead that is holding below the Dalles will spawn upstream or downstream of that location is not relevant to its exposure to fishing pressure, regardless of whether that fishing occurs in the upper river or middle river. The distance involved is not great in terms of a steelhead's migration potential, and there really isn't an appreciable difference in spawn timing in the upper and lower Skagit rivers - Mar 15 - June 10 with the peak around May 18. The Sauk, however, peaks roughly 2 weeks earlier than the Skagit.
The main difference with the fishery restricted as it is, is that steelhead that spawn downstream of the Dalles, and enter the Skagit after March 15, don't get fished on, except by plunkers in the lower river - which is a pretty fair angling rate in some years.
The upshot, in my opinion, is that the steelhead productivity advantages previously alluded to of alternating the location of the CNR season simply will not acrue. However, it would provide a change of scenery, which can be nice, as I enjoy fishing many, many places on the middle river, and never cease to be amazed at the odd places I've found a good piece of steelhead holding water.
Sincerely,
Salmo g.