Like Brad says above, there's no way to configure the RichTextField's HTML Purifier instance – but you could easily create a custom fieldtype that instantiates HTML Purifier w/ different options.
First, scaffold a custom plugin using pluginfactory.io. Name the plugin what you want, don't enable any components.
Second, create a file called
/fieldtypes/MyPluginHandle_RichTextFieldType.php inside your custom plugin's directory (replacing
MyPluginHandle with the actual handle for your plugin, of course).
Third, paste in everything from
craft/app/fieldtypes/RichTextFieldType.php into your custom fieldtype; then remove everything except the
getName() methods. Make sure you're returning something different than "Rich Text" from the latter method, and configure
\CHtmlPurifier() according to your needs inside the
Finally, install your plugin and convert any existing Rich Text fields to your custom fieldtype.