Heather:

Not even sure if this is possible without some testing but;

You would need to create a custom Adagio (Crystal) Report using the customer current transactions to determine outstanding invoices and the tax tables (from whatever invoice module you are using) to find the HST. The report output would be your customer statement (it would be possible to dress this up to look exactly like a statement).

I would think this will require a fair amount of expertize in Crystal and the Adagio database.