get_supplied_options() should work fine both in UIs and Plugins. My guess is the host only gives the sampleRate option to the Plugin and not the UI. Which host are you working with?

Also, you can compile the UI with a -DLVTK_DEBUG=1, and the console will print the validation status for any features passed in by the host. You could also from the UI send a patch get message and make sure the Plugin knows how to respond appropriately.