Hi John 

have a look at this method _apply_program_reward in sale.order model under the module sale_loyality

but since this method is not public you cannot access it throw XML-RPC so you may develop a public method that call that one after

For the front end odoo has an endpoint that you may also use, this endpoint will apply the coupon, this can also be useful

@route(['/coupon/<string:code>'], type='http', auth='public', website=True, sitemap=False)
def activate_coupon(self, code, r='/shop', **kw):

hope this helps

Daniel