Tokyo to Hong Kong

Part one:

Yes, you are allowed to do that, but there is one problem. You have mentioned that you will have a proof of the booked train tickets and this is also fine, but where you will collect your real, paper tickets if you don't make a booking of some accommodation in Guangzhou? Just to inform you, you cannot use a high speed train from Guangzhou to Hong Kong, but only a through train departing from Guangzhou East railway station and arriving to Hung Hom railway station in Hong Kong. Also, you are not allowed to book the ticket for this train on its original MTR website because they do not offer an e-confirmation with a name on it. So, you have to book your ticket for this train through some other agency like website like this one (Travel China Guide) that will offer you an e-confirmation with your name on it. Anyway, they all have a policy of delivering a real, paper ticket on the address of your choice, usually at the address of your accommodation. To offer an address of your accommodation, you have to make a booking and here we have the problem that I have started with my entire post. This option is fine for those who intend to have at least an overnight in Guangzhou, but very inconvenient for those who intend to travel from Guangzhou to Hong Kong on the same day.

Part two:

As an alternative, I would suggest you to take a ferry from Nansha port (south from Guangzhou and reachable by metro from Guangzhou airport - with a change or two) to Hong Kong. You will also have to have an e-confirmation, but this is much easier because when you book your ticket through CKS - Chu Kong Passenger transport website, you will receive an e-confirmation with your name. Also, you will not have to bother with the delivery of the real, paper ticket because you will collect your real, paper ticket at Nansha port, upon your arrival there.

Your accommodation in Hong Kong is of no interest for Chinese immigration.