I don't have a concrete answer for you, but I can tell you where I would start. I warn you that this line of inquiry is not for the faint of heart.
The style information for the aero theme under Windows 7 is located in a resource-laden DLL called shellstyle.dll, in C:\Windows\Resources\Themes\Aero\Shell\NormalColor. The file's resources can be viewed with a tool called Resource Hacker, available here. Upon opening the file, you should browse to /UIFILE/1/1033 - and you'll find the XML definitions of the styles used by Aero, as shown below. This should include most of the fonts, sizes, and icons used throughout explorer. CSS Definitions are in /23/CPWEBVM.CSS.
I've successfully edited this resource before to remove a number of UI elements I didn't like in explorer, such as the Command Bar. If the font for taskbar entries is defined anywhere at all, it will be in this file. It's also possible that this font inherits from some global value and can be overridden in this file - there's actually a good deal of flexibility, but you need to know what you're doing.
If you want to make changes, back up the original file, take ownership, and replace it. Then you'll need to manually restart explorer.exe to see the effects. If you have any doubts whatsoever about editing this file, I would leave it well alone and learn to live with the existing fonts in the taskbar buttons.