does anyone know if it's possible, using billing schedules, to get the correct billing dates on the invoice line item columns? currently it just looks to copy from the sales order, which isn't accurate for, say, quarterly invoices

You should be able to create a billing schedule with a "custom" Recurrence Frequency. Underneath the main section, you'll see a table of what the custom schedule should look like. Use "3" for the count, "Month" for the units, check the box for Relative to Previous, you should be able to see the date on the 4th column, enter in a figure amount or a percentage under Amount, and whatever your payment terms.

You mean invoice copy date from sales order not from billing schedule.

Thanks to you both - not quite what I was looking for but I think I have determined that there is no way to display the correct dates on an Invoice without a script of some kind.