As much as I would like to post the court documents, I have yet to find them in their entirety on the web.
However, the pertinent quote is referenced in "California Jurisprudence," by William M. McKinney, 1922 at footnote #16.
It can be found here at the bottom of page 744 and the beginning of 745.
I couldn't find it either, but I did find this synopsis.
http://www.jrank.org/cultures/pages/4298/People-v-de-la-Guerra.html
As a final point, Kimberly asserted that the court's interpretation of the treaty's Article IX would conflict with the California Constitution. The latter, he argued, discriminates by race while Article IX does not; only white male Mexican citizens were allowed to be state electors under the California law. The court saw nothing wrong with this potential conflict: “The possession of all political rights is not essential to [United States] citizenship….[I]t is no violation of the treaty that these qualifications were such as to exclude some of the inhabitants from certain political rights.” Put another way, the court recognized that California, as a separate political entity of the United States, had the power to grant different rights to its citizens than the federal government might.
Seems to me that a citizen of any particular state is also a citizen of the United States, and that a state may grant different political rights to its citizens than the federal government does.
I don't see anything that would indicate, however, that state citizenship would have a darn thing to do with federal taxation.
I do believe that the "voluntary" notion of taxation has to do with the idea that the activity that results in taxation, work for example, is voluntary. In other words, if I wish to avoid federal taxes I may choose to keep my income below taxable levels. It has nothing to do with citizenship.