What you are trying to do sounds very similar to how a 'selected' player gets highlighted in the leaderboard example.
Since both your helper and your mouseover event have access to some context from the template calling them you can make use of that information.
// new event
'mouseover .hover-check': function () {
Session.set('showHover', this._id); //can be any field in the article that is unique between articles
}
//new helper
Template.articleItem.helpers({ //fix typo in helpers
showHover: function () {
return Session.equals( 'showHover', this._id ); //again I use _id but just match it to whatever you set with mouseover.
}
});
And in your template your idea of using an #if block seems correct:
//in articleItem template
{{#if showHover}}
extra content...
{{/if}}
It may also be easier just to include {{showHover}} in the articleItem template and then instead of return a boolean from your helper, return whatever it is you want to add. Text, a subtemplate...