Don't think so, not in this store.
Back in the early days, free downloads counted like paid downloads; some of the apps in a good position in the category rankings today got there that way, and those old downloads still seem to count like purchases, except in Top Paid.
But a couple of years ago they stopped that, so when you have it free, you can't switch it back to paid and suddenly be up in the rankings -- nether in Top Paid, nor in the category AFAIK.
Nearest thing to a 0$ promotion may be this: some devs offer a money back guarantee with no trial, and then pay that refund directly, so all the returns still count as sales in App World. The 30% lost would be like advertising cost. This has proven to work for apps where the dev created strong appeal in the description, so people felt they just had to try it -- and even though most of the sales may have been refunded, the app was still catapulted to #1.
