For future posts, I'd like to suggest that if you do find an answer or solution, you post that information as a reply.  This way, others who are struggling with a similar issue will be able to use the information you posted to help them. 

My recommended solution for this problem is to use the UltraCalcManager.  It not only integrates into the WinGrid, but also allows integration into other fields such as text boxes.  The UltraCalcManager uses Excel like formulas, and can take inputs from components including the grid and textboxes, as well as output the calculation result into the component/property of your choice.

hope this helps,

-Tony

I would like to be able to reference a summary value as part of the formula. For example, I want to calculate the percentage of a cell value compared to the sum of all cells in the column. I know that in this example, I can just use a formula like "[mycolumn] / sum( [mycolumn(*)] ).  But I also have more complicated cases where the summary is a custom summary calculator where I can't use a workaround like this.

When you add your summary, you must specify a Key. There are a bunch of overloads to the Summaries.Add method that let you specify a key. Or you can just get the SummarySettings that is returned from the Add method and then set the Key. 

Once your summary has a key, you can use that key in a formula in the same way you would reference a column.