If you're using Fusion to get to Adjust, you should be returning a copy of the image to Lightroom, so the original image should still be available in the stack to copy and paste the metadata from. Fusion is a very basic plugin host/editor, and while it's a workaround for Lightroom, it's actually intended for people who don't have any sort of an image editor installed (point-and-shoot types).
Note that Topaz is releasing a new product (should be this week or next), photoFXlab, that is more Lightroom-friendly. The release price is $29; the retail price after release will be higher ($79, IIRC). I've only seen the sneak peek, but it's supposed to act like a LR plug-in from Lightroom rather than as an "edit in" option, and provides a layering/masking environment while the image is in the plugin. It will still work as a stand-alone, as Fusion does, but as far as I've been able to determine it's supposed to work directly from LR as well.
ADDED: After a glitch in the 1.0 version forced a hold (a new product registration system left some users unable to use the software), photoFXlab 1.1 has been released (today — June 29, 2012) with the old product key registration system, the "social" features removed, some genuine UI improvements, and I can confirm that it maintains metadata when round-tripping from Lightroom. The $29 launch price has been extended to July 31, 2012. (I must really be starting to sound like a shill, but I've been bugging the good folks at Topaz about this for a while, and I'm kind of impressed with the improvements they made while fixing the registration system. It feels like more than a dot release compared to 1.0.)