Set up a template variable of type textarea (I named it "relinklist"), add your substitution list into the default options area, select widget type delimited text (with the delimiter set to ",") and assign it to the templates you are using.
I linked the plugin to the "OnParseDocument" but there may be a better event to use (since it seems to get called a couple of times, in my test case).
Anyway, then change the code to deal with the TV like this :
//$keywords = "list of keywords was here";
$keywords = $modx->getTemplateVarOutput(array("relinklist"),$docid='[~id~]',$published=1); //MattC
echo $keywords['relinklist']; //MattC inserted for debugging
$array = explode(",", $keywords['relinklist'] ); //MattC
Hope this helps,
MattC
EDIT : Note this is for Evo ...