I am not sure if this will help you, but you can highlight multiple areas of text (within a document) that are not next to each other by selecting the first piece of text, then hold down the CTRL key and select any other pieces of text as required. Then you can do whatever you want to with them - apply the highlight, or bold, or italics, or delete, or whatever. I use this one a lot. Even if it doesn't help you with this problem, it's a good shortcut to remember for other times.
The only other thing I can think of is to create a macro for it.
I did a google search and came up with the following. I don't know if this will work but you could try it.
http://www.techtalkz.com/microsoft-office/66029-there-keyboard-shortcut-highlighting.htmlRE: Is there a keyboard shortcut for highlighting?
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There is no predefined shortcut for Highlight but you can easily assign one:
Select Tools > Customize and click the Keyboard button.
Below Categories, select "All Commands".
Below Commands, select "Highlight".
Enter the desired shortcut.
In order to make the shortcut available to all documents, select Normal.dot
in the "Save changes in" list.
Click Assign.
Click Close.
To save the change to Normal.dot, press and hold the Shift key while you
select File > Save All. If a dialog box opens, asking whether to save changes
to Normal.dot, click Yes.
The shortcut will apply the highlight color that is currently selected in
the Highlight icon in the Formatting toolbar.