It may be beyond your level of interest to try, but I've often experienced that sites who put dumb validation rules in the browser will therefore omit the valid

It may be beyond your level of interest to try, but I've often experienced that sites who put dumb validation rules in the browser will therefore omit the validation on the server. Thus, you're one well crafted curl request away from unsubscription bliss. Chrome will also give you some help in that its network request list in the developer tools has a "copy as curl" in the right-click menu.