Get the Silk details that you want for the assembly displayed, don't display anything else. From the menu, File>Export>Sub-Drawing, check the Find Filter just has Text selcted (and lines, if required), window select all the items in the design canvas and specify an origin - you could specify "x 0 0" at the command line for 0,0 - name the CLP file generated. Go to Setup>Subclasses, pick a class, like Manufacturing, and add a new subclass, like AssyRefs. Open the CLP file generated with a Text Editor, locate some RefDes items to see the Class / SubClass, likely REFDES/SILKSCREEN_TOP for example, use the Replace text function to replace the REFDES/SILKSCREEN_TOP values with MANUFACTURING/ASSYREFS (whatever you called the new subclass), repeat this for any lines as well. Back in PCB Editor, File>Import>Sub-Drawing, pick the CLP file and specify the origin, it's easier if you used 0,0 previously because you can type "x 0 0" at the command line again to get the exact location! The subdrawing data will now be imported to your new layer. You probably want to try this with a copy of the data initially.