There's a very short way to do that directly from LyX: just add a short title ("Insert>Short title" while the cursor is in the slide's title) and type "titlepos=" for every slide you don't want the logo to appear in. Then, the short title may easily be pasted onto several slides...
You have a very good point. I wasn't sure whether logopos was available for slides too and didn't really have time to think about it, so presented the {\pdsetup...} solution. But indeed, \begin{slide}[logopos=]{Title} \end{slide} is the way to go for disabling the logo for a specific slide. And it's easy to do from LyX as well. Thanks for the reminder, best, -Hendri.