Exporting the mask works great for doing pixel shift without motion correction and then blurring the chequered artifacts away. It looks close enough to motion blur which is fine unless the trees are the main subject.
I just used gaussian which might not be a perfect result for motion blur but it's a simple and quick solution. This is obviously no good for the speeding van type motion but for foilage and shadows its pretty decent.
I realized that using the mask to blur migh be doable in Rawtherapee rather than exporting and gimping? I'm sure some sophisticated computed motion blur is possible but gaussian seems to do the job.