Would development of Vassal 4 or later be simplified if all the calculations required by module developers were handled externally to vassal and hyperlinks used to get those results into vassal itself?
When I was working, I needed to write reports based on a bunch of calculations. Excel did the calcs and word did the reports. To make my life easier, I developed a word template that 'imported' the appropriate data from the various excel cells, including any graphs, etc. All I had to do then was write words. The excel data was there.
Excel and its various equivalents, can handle a lot of formulas. Global properties, equations, etc can all be handled within the spreadsheet. Vassal would just access the spreadsheet automatically, either to get a result or to pass data to get a result.
Just a thought.