Hi George,
Welcome to the world of Xcelsius Dashboards!!! 
What is the version of Xcelsius (Dashboard Design) and SAP BusinessObjects Enterprise (SAP BI Platform)?
Document Download URL Syntax varies for 3.X and 4.X version:
- Document download URL for 3.X:
http://<WebApplicationServerName:WebApplicationServerPort>/Xcelsius/opendoc/documentDownload?iDocID=<CUID of child dashboard>&sIDType=CUID&sKind=Flash
- Document download URL for 4.X:
http://<WebApplicationServerName:WebApplicationServerPort>/BOE/Xcelsius/opendoc/documentDownload?iDocID=<CUID of child dashboard>&sIDType=CUID&sKind=Flash
Alternatively you can use simple relative URL syntax(no need to specify whole url):
documentDownload?iDocID=<CUID of child dashboard>&sIDType=CUID&sKind=Flash
Quick suggestion, you can test download document URL with some simple child dashboard and then implement it on large swfs. No need to format excel cell as hyperlink.
You may refer following threads:
http://scn.sap.com/thread/3210636
http://scn.sap.com/thread/3193444
Hope this helps!
Regards,
Vinay Mhaske