It's possible to decorate the form component's (form:components/form) formProcessor configuration node but that decoration is ignored when the processors are executed by the method AbstractFormEngine#executeProcessors(). The problem is the TemplateDefinition provider is not being used.

Targeted config:
/modules/form/templates/components/form/formProcessors

Using the Definitions app you can see the form component template is being decorated. Decoration is technically possible but the AbstractFormEngine needs to get the processor list from the aggregated template definition and not directly from JCR.

This is an important issue for those using cloud 1 because you want to be able to create complex forms, use JavaScript for processing and configure the processor with decoration rather than bootstrap.