[development] formsapi question: saving additional fields' data

We can easily add fields to existing forms. Do we have a way to save their data? Example: I want to extend blocks' configuration pages. I can form_alter where $form_id = 'block_admin_configure', adding new fields. But block_admin_configure_submit() won't look for or save the added fields. To do so, I can only think of (a) (mis)using the #validate attribute to specify a function to pass the data to and save them there, or (b) hijacking the form by changing the form_id and sending it to a custom _submit function, which saves the custom fields and then invokes the original _submit function. Better approaches? Tips? Solutions that people have used elsewhere? Am I overlooking a relevant Forms API attribute? Thanks.