I totally agree that calculating isomerization factors below 212* is a guessing game. I believe most brewers would agree with the quote you provided. There are also some brewers experimenting with with "zero IBU" beers where all hops are added post boil with majority being sub 170*. From 212* to 175* alpha acids will isomerize, but even below these temps these "zero IBU" brewers suggest that significant whirlpool contributions still provide perceived bitterness.
Anyway, as stated previously, I'm not concerned about calculating IBUs perfectly so much as being able to separate the two post boil processes to keep my brew session organized. From 212 to 175 some isomerization will occur and it would be nice to capture that, at whatever ever hop utility % used.
I'll play with it a bit as you suggest. I'm thinking I'll set utility factor to 5%. I typically do equal parts of the same hops at FO and at WP. From FO to 170* takes only 10 minutes so there's not a significant contribution here.
As more and more of us brew NE IPA style beers where there is an emphasis on post boil additions, I think BeerSmith will see more requests to either adjust Hop Utility Factor for each hop addition or to add an additional Whirlpool option.