Yes, as you are flying First Class, you should be telephoned by the First Lounge on the day prior. As you say, you may be uncontactable, meaning that you may have to enquire about a Spa booking as soon as you arrive at the Lounge. The treatments can book out.
I do not have a telephone number for the First Lounge, but the Business Lounge is 03 8336 4294 (+61 3 8336 4294). They may be able to transfer you. The Spa hours are generally 7.30am to 4.00pm daily.
As for PJs, I’m not sure about daytime flights. If they are not offered, I would spin the FAs a bit of a story, as they may have some stored on-board anyway.
So, if you are not able to pre-book the Spa, try to arrive early, enquire about a booking, eat, drink and have some great relaxation time. Enjoy.
Called the biz lounge and was transferred to the first class spa immediately. Booked the treatment for my wife, and they were extremely thorough and helpful. Thanks for the assistance!