Well I can expand here on what the features of both programs are in relation to multimedia.
Both products you can set up unlimited categories to upload products or photos to.
Reviewpost is setup to allow up to 6 images to be uploaded to represent a single product. No multimedia capability at all
Photopost you can upload a multimedia file and set a switch to allow the multimedia to be played inline as represented here in this link as opposed to having them served as download links.
http://www.reeftalk.com/gallery/show...t/500/ppuser/1
You would be able to add an .xml extention in the multimedia types in admin => upload options and be able to upload an xml file however our application is not going to present it as a downloadable multimedia link for a user to download.
Now given the functionality above of Photopost with multimedia files this would probally be a better choice than Reviewpost in your situation.
Any customer is free to modify the base code to suit there specific needs should there be features not present in the software that they need. As to any specific changes you need to do I would not know.