The Section Partial Messages gives an overview of this script. Here is the storeparts script (you might want to open it in a separate browser window). To install it, see the Section Programs in This Book's Archive.
The script is simple. It extracts the Content-type: header field from the message and gets the id= information from the field. Then it uses pick to search for messages matching that id. pick stores the messages in a sequence named temp. You might want to customize storeparts to use a different sequence name -- or to accept a -sequence option on the command line (and, if you want to be really fancy, have it check your MH profile too).