Or maybe, one workaround could be, that there will be some hidden field in the form, which would say, if it is valid.
Then on some event (perhaps on validate on whole form) there could be a check, if this hidden's value holds actual state of form validity and if not, it would be switched. It may not be a good idea to set the value all the time if the form should be signed (I don't really know, how this works).
Then on server, the only validation would be reading of this hidden field.
For me it should be good enought, what do you think about this, are there any hidden gotchas ?