Unfortunately there is nothing out of box that allows this functionality. JS is the best route to accomplish this.
Set the value of your Select All checkbox to a single asterisk (*).
Then use this existing code: MktoForms2 :: Select All Checkbox. It'll automatically pick up all the Select All checkboxes on your form without having to know any field names.
Unfortunately, using some pure DOM events (generally frowned upon) is necessary here, as we can't solely rely on the Marketo Forms API events.
Ryley, Have one of the options be "all of the above". Then have a smart campaign in the background check all those boxes on the contact's record if they choose the "select all option". Hope that helps!