Ian,
I think it is a great idea. I've only played a handful of sub-70 par courses, but they include some of my all-time favorites. I see no reason why it wouldn't work, particularly as the 3rd full 18 on site.
My question for you (and any of the other architects that care to answer) is - if you went into the design process with a goal of having a par 68 (or 69) course, in what ways would you seek to accomplish this and "how far" would you "push" to get a sub-70 final product? For example, let's say after walking/studying the land and working on finding the best routing and holes (I don't pretend to know what really goes into that), you discover there are many natural spots for par 5s (or something along those lines)...would you consider trying to "convert" those holes into very long 4s instead? This is just one example of what I imagine you might consider, but hopefully you know what I am getting at. I'm sure the decision-making process would be far more nuanced than my questions might suggest. Ultimately, I am wondering how "certain" you would be going in that you'd finish sub-70, and whether you think there could be significant compromise (vs. what you deem the best possible course the land would yield).
Thanks!