This would be very interesting to see.
Unless this is supported, you deploy 2 instances of blesta, one on the sub domain, while the other one on the main website link /order and set both of them to use the same databse as the backedn, so that everything is the same.
Then have a scenarios where on the website everyone is sent to the order side, while you give the clients the client.domain.com for their login area. Unless there is a better way for that.