Update:
I think I'll sort of work around this by creating a suppress formula for each of the 12 periods. I probably will have to rethink this in terms of totals because suppressed values don't print but they do summarize.
Create two parameters:
StartDate
EndDate
For each of the 12 fields on the report I'm either going to suppress them using conditional or create a new formula that reports ""NA"" (or whatever I can stick in there that won't cause an error when summarizing).
The user is aware that this won't be a clean cutoff -- meaning your start/end date is defining an entire month.
The formula that I tentatively worked out (right now I'm sticking this in a conditional suppression for testing purposes but I suspect I may have to code formulas for each month due to the need to total and suppressed values continue to total which is not what we'd want).:
date(toNumber({IM_SalesHistoryWrk.FiscalCalYear}),10,15) < {?StartDate} or date(toNumber({IM_SalesHistoryWrk.FiscalCalYear}),10,15) >{?EndDate}
And I'd have to create this formula for each of the 12 columns and change the coded date to advance by one month (so the next column date would use 11 as the month, etc)