I ran into the same issue.
The problem is that, although ConverterGroup will chain a series of IValueConverters, and MultiConverterGroup will chain a series of IMultiValueConverters together, there doesn't seem to be any way of chaining a series of IMultiValueConverters into an IValueConverter.
I finally used a workaround of a MultiBinding with my IMultiValueConverters into a Tag property and then binding my real target to the Tag using an IValueConverter.
I think MultiConverterGroup should have a Converter property which takes an IValueConverter and applies this to it's result before returning.
You could even then populate this property with a ConverterGroup and get another chain going!
In fact I think I will raise this through the issue tracker.