I do not have the mapping toolbox to test this with, so I can only suggest a possible workaround:
If you create a function (in its own .m file) that accepts an optional parameter (which you can ignore), and the function returns the colormap you want, then try giving the quoted name of that function in place of the regular colormap string name.
The optional parameter for the function will be (if it is present) the number of entries desired for the colormap. 'jet' and so on do not return static information: they calculate the colormap suitable for the number of entries requested (default 256)
function cmap = MyColorMap(varargin)
cmap = [...