I found a (DIRTY) solution myself.

If you are willing to sacrifice the "invoice" function you need just to install the report_to_printer module and edit a qweb report and the js script (point_of_sale/static/src/js/screens.js) to archive the result.

Let me explain:

As you probably knows, the report to printer let you use any printer configured in your system, under cups in linux in my case.

After that you just need to change the content of the template on invoice report (point_of_sale.report_invoice) and the relative qweb report associated. Done! Now you can print any qweb report you lke directly from POS without a proxy printer.  

If you want to edit the pos layout to pull out the validate button and have just the modified version of the invoice button, you need to edit the script "point_of_sale/static/src/js/screens.js".

Pulling out (or remark) the "validate" function

this.add_action_button({

label: _t('Validate'),

name: 'validation',

icon: '/point_of_sale/static/src/img/icons/png48/validate.png',

click: function(){

self.validate_order();

},

});

and modify the invoice part to suits yours needs:

if( this.pos.config.iface_invoicing ){

this.add_action_button({

label: _t('Invoice'),

name: 'invoice',

icon: '/point_of_sale/static/src/img/icons/png48/invoice.png',

click: function(){

self.validate_order({invoice: true});

},

});

}

That's all.