The reason for the script not supporting the TEXTAREA element is because large amounts of text may be entered into it. The script relies on JavaScirpt cookies to save the information entered, which has a size limit of just 4kb per domain (all cookies added together). So it's not a good idea in general to use this script on textareas unless you have a way of enforcing the amount of text that gets entered by the user.
Having said that, you should be able to disable the script from checking the field type by removing the line:
entirely, plus the part in red in the below:
if (cookievaluepair[i].split(":")!="toggleboxid" && this.getfield(cookievaluepair[i].split(":")).type =="text" )