Okay, I've found a solution to this myself. I may as well share it here for anyone else who might come along later with a similar problem:
The trick is to insert a transparent picture, about the same width and height as the word you want to be hyperlinked. I've included one here that's suitable for the word 'Contents' in the font Tahoma 12pt. The size isn't really that important, though, as you can use Word's picture grip handles to resize it to your liking if necessary.
Obviously, it's all transparency so it's invisible, but right-click to the left of the white space above this text and you'll see it is indeed an image that can be saved.
Once you've saved the image, do the following:
- Edit the footer and type in the text you want to link, formatting it in the 'Hyperlink' style if so desired.
- Insert the picture into the footer – being careful not to click outside the picture until you're finished, as it's really difficult to select again if it gets deselected.
- Right-click on it and set its text wrapping property to 'Behind Text'.
- Right-click on it again and set its hyperlink property to point to the location you want.
- Use the grip handles to position and size the picture so that it exactly covers the same area as your text.
- Close the Header & Footer editing tool.
One last thing: unlike the other links to bookmarks in the finished PDF document, for some reason this one displayed a tooltip detailing the link location. The only way I could find to get rid of this was to go to "Office button | Word Options | Proofing | AutoCorrect Options… | AutoFormat" and uncheck the option "Internet and network paths with hyperlinks" under the "Replace" header.