George,
The TF38 attacks July 15 on Otaru/Ishikari you mentioned would show that the navy strike map I found is incomplete and there were navy attacks close to Sapporo.
It think it is possible that there may not have been any B-29 attacks on Sapporo or even Hokkaido during the war.
This is an article about the "Longest" B-29 bombing mission which went from Guam to Akita:
https://www.pacaf.af.mil/News/Articl...ended-on-guam/
Akita is in northern Honshu.