I would not be surprised if the existing LP Pro support in Ardour 8 works with an older version, but they are fundamentally different devices.
I can check on it whenever I get started on the mini and X versions.
I definitely don't have the budget for a Mk3 but I can see a number of original Launchpad Pros and Mk2s for sale on local websites for not much money. Count me in as another person interested!