Shimano supplied a steel mushroom-shaped plug (TL-FC15) that filled the hollow spindle of their Octalink bottom brackets and allowed a standard square taper crank puller to work. I expect any LBS should have a couple lying around. If you can't get one locally, Bike Tools Etc. has them for $5:
http://www.biketoolsetc.com/index.cg..._id=SH-TL-FC15
You may be able to improvise with the proper diameter steel washer. As a guide, the top cap diameter of TL-FC-15 is 0.650"