I put an Alfine hub into my Singlecross (the singlespeed version of the Specialized Tricross cyclocross bike). While using a Nexus8 would be easier than an Alfine, it's not nearly as trivial as you are making it sound. You can read about that
here.
I don't know what the new SA IGHs look like, but the one i'm familiar with only adds gear ratios above the one provided by the chainring and cog, so for starters you'd have to put a much smaller chainring and/or larger cog on to have low enough gears. You'd also need to "respace" the hub unless you start with a frame with mountain/hybrid dropout spacing (135 mm). The good news is that respacing would also reduce the impact on your chainline, perhaps making a new BB unnecessary.