Dear Leonardo, sorry to see that your very interesting technical question has not yet received any expert answers. To the best of my knowledge, the classical synthetic protocols for preparing rhodizonic acid from inositol always use nitric acid. For example, please have a look at the following detailed procedure:

Potassium rhodizonate

http://www.sciencemadness.org/smwiki/index.php/Potassium_rhodizonate

Even a tutorial video has been published in which this synthesis is illustrated:

Make Potassium Rhodizonate (for lead testing)

https://www.youtube.com/watch?v=BLmycZ2nrt0

If you run this preparation in a well-ventilated fume hood on a relatively small scale (e.g. 10 grams) this should present no real danger. As chemists we should know how to safely handle toxic and potentially dangerous materials. By the way, rhodizonic acid is also available from commercial sources (just in case that you need only a small amount).

Good luck with your work and best wishes, Frank Edelmann