Hi Jeff,
I would use Photo Mechanic's Convert RAW to DNG utility in the Tools menu to create new DNG files with full sized previews. You will need to have one of Adobe's recent DNG Converter apps installed before you can use this utility on your Topaz Photo DNG files.
The new DNG files can be cropped in Photo Mechanic. In the Preview window use the x or c shortcut keys will toggle the Crop tool on and off. To copy the Crop, use the Command-C shortcut. To Paste the crop use the Command-V shortcut on the next Preview image. If you are using a PC, then use the Control-C and Control-V shortcuts.
If you need to batch apply the crop to the selected group of thumbnails, first copy the crop in the Preview window. Now go back to the Contact Sheet and select the files for the copy update. Right-click any of the selected thumbnails and then go to the Crop option and then the Paste Crop option. I hope this helps.
Best regards,
--Bob