This is more a function of proration than package term. Nobody wants to get an invoice every hour.
I think the way this would work is that you'd enter the hourly price in the package, after hourly billing is enabled through some option there. You'd still have a term of say 1 month, and the customer would be charged hourly rate * 24 * 30 for the month. If they cancel early, a prorata credit refund of the remaining hours could be made.
EDIT: Alternatively, we do nothing different in the configuration, other than to display the pricing on the order form as hourly. So, you'd save the price for the full term, and it would be converted to show the hourly price during checkout.
I always thought this was a bit of a marketing strategy. 25 cents an hour sounds a lot better than $180/mo.