There is no provision to write a custom form element as part of a plugin that has it's rendering template in the plugin?
My reading of the $OUTPUT mform_element function appears to suggest that the form element name is hard coded in to the form 'core_form/element-*'
We've got a custom control that we use on an mForm that I'm trying to sort out for 3.2 and it it doesn't pick up the basic form layout (i.e. label on left, control on right).