In what situations, if any, would you treat locally advanced (non metastatic) anal cancer with induction chemotherapy followed by chemoRT rather than chemo/RT by itself?

Hard to know. I would offer that it is a rare event for me to consider induction chemotherapy prior to chemoradiotherapy at this point. Part of it stems from an honest lack of faith in chemotherapy relative to the effectiveness of radiotherapy/chemoradiotherapy, which has complete response rates in the 90% range. 

When we look at the data for metastatic disease, whether it is cisplatin/FU, mitomycin/FU, or other compounds (there are data for taxanes, ifosfamide, cetuximab), there are few to no complete responders in this setting. I would have a hard time deferring the curative intent regimen, however toxic, in favor of an induction chemo regimen unless there were some reason that radiotherapy would not be an option. 

Very few scenarios. Utilizing neoadjuvant chemotherapy would likely not change the radiation field.