I've always wanted something like this too and it has been requested before.
In the mean time, as a work around I edit my "license.txt" file that is stored in the root of the flash drive. Then in the unRAID webgui, if I click on the "Tools" menu item and then click on "EULA", I can read whatever I have written in the "license.txt" file. The license file is a .md or Markdown file and can use special formatting for hyperlinks, titles, bulleted lists, etc. Here is a link with a template and some examples that are often found in a markdown file. https://gist.github.com/PurpleBooth/109311bb0361f32d87a2 Remember to keep a backup of your custom license file as it will get overwritten on any upgrades to the unRAID OS. Here is another link that shows you a live and editable markdown file that you can use as an example to create your own file. https://www.makeareadme.com/
Another option might be to install the "Custom Tab" plugin by Squid and have a link to a page you create on the flash drive. I have not tried or tested this method but I think it might work.