Which I tested, and works just fine. You can manually create checkboxes for each item, but that wouldn't be the result you wanted, I think.
It turns out that there is a different bug related to determining when to use append the square brackets ([]) and when not to. The code above will work for now, but you'll want to change it to this with the next update.
So you don't have to worry about modifying this, I've attached the updated htmlform extension.
HTMLForm also accepts a new option, defaultValue, which lets you set a default value for custom fields. It's a bit of a hack, because specifying value overrides the form-saved-with-errors method to re-fill in changed values.