In answer to "would it get used", I believe it would get quite a bit of use. We have a problem locally in that people new to cycling, which is most of the people riding, can do a few miles on bike paths and such, but have trouble finding routes to ride that allow them to move to the next level since Eugene/Springtucky are surrounded by a small (~10 mile) ring of aggressive suburbs. Thus, in order for people to get to the good stuff, they have to learn how to ride amongst aggressive drivers on roads that often require vehicular cycling skills that they just don't have. I'm not sure how many would choose to ride the whole trip, but most people are somewhat goal driven and once they start up they will set a goal of eventually doing it all. I would expect to see many hundreds of people each and every sunny weekend day if such a route came to be.

Now, about that "it". Your idea is great, but perhaps step one (or step two) should be a rail-with-trail along the tracks (parallel to hwy 58). This would make the journey go to Oakridge, which would create a natural stopping point (and money spending break) and perhaps garner some support from the county commissioner representing that area. Getting to McKenzie pass would then involve the Aufderheide, which is almost as nice, and there are back roads to Cottage Grove from there as well. There could be some glorious loops to be had here, but getting something parallel to hwy 126 and/or hwy 58 is/are really good ideas.

On a related note, Lane County has been putting up grant money for tourism ideas/projects the past couple of years. A route such as you suggest, especially if coupled to an organized ride or two, could bring in tourism money that might be shown to pay for it over just a few years. I'm no good at crunching such numbers, but if you know someone who is it is possible that the commissioners might be inclined to support doing the small amount of work needed if it can be shown to have economic benefits. It can't hurt to ask, especially since Bozievich actually rides his bike on occasion and will likely "get it", and Peter Sorenson would vote per his constituents' pro-bike stance. Thus, you only need one more vote (that's why showing people spending money in the East Lane district would be critical).