Is there an easier, script-based way to do this, please?

How to save a custom thumbnail for a library preset in Poser:-

1. Save your preset to the Library.
2. Right-click on the new preset, choose "Show in Explorer".
3. Right-click on the terminating folder name in which the preset was saved, Choose copy as text (e.g. C:UsersPublicDocumentsPoser 11 ContentRuntimeLibrariesMaterials).
4. Make a render of your scene or prop or material at 91 x 91 pixels, and save the image as .PNG. To located the old thumbnail, paste in the folder path you copied in step 3, locate the existing thumbnail and over-write it with the new render.

I purchased Thumbnail Designer script a while back, but have never got it working in Poser 11.

yes, you are working too hard!
Secondly, icons can be up to at least 256 square now... There is a little bit of prep work, but this is 2 step process. A. Create a Render Setting to create icons. Include the size you want. (You'll probably have to set this every time, and you'll need to reposition the item into the render area.) B. Control G to turn off the ground plane. Gives you a clean white background.
C. Save the new item to the library
Make the icons.
1 Render the item with your pre-made Render settings.

1. Save to the folder it lives in from Poser, overwriting the icon Poser generated for you.

I only do 1 & 4 and my thumbnails are 203 px.