I like your idea! There are many cases where a mapper thinks a way that is already mapped or he is about to map may not be practical for use by some map users, but doesn’t have/doesn’t know how/can’t be bothered to add information on what it is that may make a way impractical for use. I’ve been playing with the idea to introduce a number of new values in the highway=* space to temporarily tag newly mapped highways for which information is lacking to be able to give it a definite existing set of highway and descriptive tags, and proposed one for dangerous paths. But since most mappers seem to abuse the access=* space for this, it may not be a bad idea to introduce one or more tag values in that space to accommodate this. However, I think it should be clear that such tags are meant to be temporary, and should eventually be replaced by tags that describe what it is that makes a way impractical for some use (smoothness, sac_scale, width, etc.).
I think this is the utopia we should strive for: it is the map user that ultimately wants to decide for himself if he wants to use a way or not. We should not take that decision for him, but provide him with the information needed to take that decision himself. An ideal router would then have a number of default profiles that a user can choose to help him take the decision (GraphHopper already has several), and allow the user to create custom profiles according to his own wishes and abilities. For this to work, a large majority of ways would need to be tagged with many different quality and other descriptive secondary tags, but we are nowhere near that. Introducing temporary tags that provide at least some information (if only boolean) and at the same highlight ways for priority surveying will make this task a little less daunting.