If you have access to jQuery in your project, you can do this.

Change the name of your checkboxes so craft doesn't look at them when the form gets posted and have a hidden input in your form that you populate using using js.

This uses jQuery but you can convert to Javascript if needed. It's not perfect but it might get you on the way :)

If you have access to jQuery in your project, you can do this.

Change the name of your checkboxes so craft doesn't look at them when the form gets posted and have a hidden input (named inline with your form) that you populate using using js.

This uses jQuery but you can convert to Javascript if needed. It's not perfect but it might get you on the way :)