The issue ended up being (I think?) something to do with an incompatibility of sorts between BGRABitmap and LazReportPDFExport. For some reason, if a bitmap is created via a TBGRABitmap object, then assigned back to, say, a TImage, LazReport sees it fine, but it will not export via LazReportPDFExport. Even if I dump the image to a file via Image.Picture.SaveToFile, then load it via TfrPictureView.Picture.LoadFromFile, LazReportPDFExport just will just put a blank image in its place on the .pdf.
However, when I dump the image to a file via TBGRABitmap, then load THAT file into the report picture object, then both the report and LazReportPDFExport see it, and it comes through on the resulting .pdf. Guess that's what I get for mixing library utilities and expecting it all to work together seamlessly...
So I have a form variable of TBGRABitmap that I used to convert the image and I just used that to dump the image to a .bmp file, which I then load into the form picture object then delete after the .pdf is exported. All working fine now.