I doubt there will ever be ease-in/out functionailty added to the Pan & Zoom tool because it's not meant for detailed editing. You place the keyframes and set the size of the window and PD calculates all the details, so it's a quick and easy way to add motion to still images.
You can duplicate previous/next keyframes which lets you hold the current settings for as long as you want. You can also add a midpoint keyframe and drag it closer to where you want to ease in/out from the current size/position, but that sometimes causes the PD-generated path to add unwanted arcs.
Like Tony said, there are Ease-in/out settings for manually making any changes you want in the PiP Designer and/or Keyframe tool. Those will give you 100% control over your zoom & pan with only slightly more effort than using the P&Z tool.