If you can apply the xmp to a single file, then you can easily copy the metadata to other files.
To apply the xmp to a single file, you select the photo and then Metadata->Read Metadata from Files (this assumes you have named the xmp file with the same file name but different extension as the photo you have selected, and placed it in the same folder as the photo you have selected)
To copy to other photos, select the photo with the metadata (the one described in the above sentence) and the select the remaining photos and then click on the Sync Metadata button near the bottom right
Thank you so much. This process has eluded me, and I've resorted to the most clunky workflow (edit in > photoshop > then individually changing file info for each image). Jeez. This is going to save me so much time. Thank you!