(Disclaimer: this comes from a guy who was raised on S-Controls and can't help himself hacking some dirty JavaScript like that every once in a while)
Not a big fan of the trigger solution to be honest.
- what will you do if you need 5 CC addresses? Make 5 fields on the Case, last one unused except for handful of clients?
- Will you handle updates of contact's email address? And deletes of Contacts as well?
- Will you always remember to take out the email of guy that raised the case (would look stupid to send it to him as "To:" address and then repeat in "Additional To")?
- What if the count of characters will in future go over 255?
As mentioned by techtrekker - I'd go with URL hacking for the standard emailAuthor.jsp page (tons of search results like here or here) + JavaScript API to perform SOQL on the Contacts. From the screencast you're probably already familiar with emailAuthor.jsp.
Check if custom button with "Execute Javascript" behavior will work for you:
{!REQUIRESCRIPT("/soap/ajax/26.0/connection.js")}
var contacts = sforce.connection.query("SELECT Email FROM Contact WHERE AccountId = '{!Account.Id}' AND Named_Support_Contact__c = 'Yes' AND Id != '{!'Case.ContactId}'"); // last AND is there because I believe you want to email Case Contact person but do not spam him/her with another copy of the email
alert(contacts); // for debugging purposes, feel free to do it properly with console.log() if you want and remove completely when you finished testing
var additionaCcAddresses = '';
if(contacts != null && contacts.records != null){
for(var i=0, len = contacts.records.length; i < len; i++){
additionalCcAddresses += contacts.records[i].Email + ';';
}
}
var url = '/email/author/emailauthor.jsp?retURL=/{!Case.Id}&p3_lkid={!Case.Id}&rtype=500&p2_lkid={!Case.ContactId}&p4=' + additionalCcAddresses;
// I'm skipping selection of email template etc - your screencast shows you already populate what you want.
window.open(url, 'newEmail');
Play with it, decide whether it looks too hacky (you might feel more confident with creation of VF page with action attribute that would craft similar url and redirect the user)...