Maybe I've learned the wrong terminology all my life because everyone here seems to be completely confused about what the term "customs" means! As far as I'm aware, "customs" is the clearing of your baggage and "immigration" is the processing of your passport. They are two completely different things! Thus all the replies in this thread are 100% inaccurate. Please get the terms correct or you are just confusing people. If your luggage goes to HKT without you touching it then you are clearing customs in HKT, not BKK. If you collect your bag in BKK then re-checkin at BKK then you are clearing customs at BKK. It is my understanding that all passengers do clear immigration at BKK, which again is totally different than clearing customs.

So I believe the correct answer is that if you already have your domestic boarding pass and you have enough time, you should be able to go up to the international ROP lounge on level 3 when you first arrive and use that lounge. I've seen no mention of any restriction by the ROP lounge for this case but have never tried it myself. Please do let us know if it works. I've seen some quite long lines at the point where international-to-domestic transfer passengers clear immigration so be sure to allow yourself enough time for that. If you do not have your domestic boarding pass and if you cannot get it before clearing immigration at BKK, then you will not be able to use the international ROP lounge but will be able to use the domestic lounge which as noted does not have showers.

Hope that clears up the confusion and please someone correct me if I misunderstood the terms "customs" and "immigration".