Hi Hal,
Based on your description, you wonder to add a List Page which products are listed in that page. And you wonder to update 300 combo boxes automatically when the List Page update. I suppose you can use CODE to get your result.
I suggest you post your issue to MSDN forum for Excel Developer, Im not good at VBA code and this forum is to discuss questions and feedback for Microsoft Excel.
http://social.msdn.microsoft.com/Forums/en-US/home?forum=exceldev&filter=alltypes&sort=lastpostdesc
The reason why we recommend posting appropriately is you will get the most qualified pool of respondents, and other partners who read the forums regularly can either share their knowledge or learn from your interaction with us. Thank you for your understanding.
Regards,
George ZhaoTechNet Community Support