So, I second all the remarks on (temporary) tempo changes, viz. accelerando and ritartando. In Finale 26 in Human Playback it seems not to work at all anymore. I tried to define one - including all the tedious and difficult Shape definition - but in the end my complete score went beserk. All of a sudden there were tempo changes in measures I never defined it in! I got in panic and simple shut down - luckily without saving!  I had to restart the computer. So I got the original file back, but still no ritardando over the three measures I intended it for.
Is there a solution? Just to define a rit. over a few (predefined) measures?? I don't trust myself to define another 'Shape'.

So I second everybody who aims to do the same thing, as pointed out. I continue without ritardando, but by no means whole heartedly.

(Windows 7/64 - Finale 26, latest update)