I'm interested in making links from an Excel spreadsheet to photos in my iPhoto Library.
You can link to a photo outside of iPhoto but Excel does not support the Media Browser feature which would allow it to select a photo from inside the library.
HOWEVER, You might be able to make an alias of the Originals folder inside the iPhoto Library package and use it to navigate to your picture and link to it. Use the Insert->Picture->From File menu option and select the checkbox to link to the photo.
It will be difficult to find the photo but a search should be able to do it. Then you can either opt to embed the photo in the spreadsheet or not at the time of insertion. I would use this method very carefully so you don't damage the library. *Do not move or rename any item inside the library package from the Finder!*
This would be only for use on your computer. Not on the web if that's what you meant by hyperlinking.
OT