I’ve created Issue 9128: Send invoices to SII - Tryton issue tracker which implements main of the desings discussed here.
The module is not complete (see the issue for more details) but it allows to send basic invoices to the tax authority.
I will like to receive some feedback about it’s desing to see if it fits all the requirements.
If anyone has any special requirement or a special type of invoice that should be tested please drop a comment so we can ensure that they desing is flexible enougth to support all the corner cases.