There was already a flag "-id" for the includelets, but it was not passed correctly (just fixed this). The handling of the css classes is more complicated, since the rendering of the includelts is governed by the "-decoration portlet|edit|pain|rightbox|none" flag.
xowiki sets already a CSS class for all includelet content depending on the class of the includelet. E.g. for the includelet "available-includelets", it uses the CSS class "available-includelets". If would be certainly possible to use multiple classes, but i have not had the need so far.
I am not sure about adding a "cssstyle", since it requires editing of the pages, when the styles are changed. It is better to use css classes....
