Your bones ache? Old timey “medicine” says, cut open a Whale, & submerge your legs up to the knees in it! But it has to be a rotting Whale Carcass. Get it right.
I knew there was a reason why RFK Jr became HHS secretary. That’s the kind of ingenuity we are looking for.
First check whale carcass for dynamite.
Back when whaling from open boats was a thing, I’ll be that one thing whalers very rarely were was warm. I’d also bet that warm baths would be hard to come by whether on ship or shore.
Even today we immerse sore muscles and achy joints in warm water, or apply electric heating pads. Only gives temporary relief, but relieve it does.
A still-warm whale carcass is just a field-expedient jacuzzi bath. And probably equally effective. Shame about the aroma.
I tried this, but got run off by the polar bears.
There are currently three seal carcasses rotting on the beach within walking distance of my house. Probably elephant seals; they seem too big to be harbor seals. Will those work? The stench is pretty overpowering even from several hundred feet away. “I thought they smelled bad on the outside.”
So I have to murder a whale and quickly get it’s still warm corpse up onto the beach with a patient waiting in a chair while I slice it open? Sounds real practical. 
[Politically incorrect joke inside - spoilered]
According to historians, the paragraph that mentions, that " … if in a bind (or in the midwest) you can also use an orphan…" got nixxed by some woke “university-educated” editor from the westcoast.
I thought they smelled bad on the outside.
I tried to get that prescription filled, but the pharmacist at CVS kept telling me my insurance wouldn’t cover it unless I wanted a generic marine mammal.
We’re all counting on you to test and report back.
I worked for many years developing over the counter pain relievers. My suspicion is that this could work as well as at least one major brand on the market in the US. The problem is packaging of the whale. Also the likelihood that clothing or bedding might be stained. Perhaps development efforts can overcome these obstacles.
It is unfortunate that it is the gas within the whale that does the healing. If that wasn’t the case, you could just pop your whale into a blender and later microwave, to the desired warmth, enough for a shallow whale smoothie sitz bath.
And keep the rest in your freezer(s) for future treatments.
Big Pharma’s cetacean division would never go for that. Where’s the repeat business? One whale lasts a village a lifetime.
An enterprising cruise line could sponsor a Whale Healing Cruise. For only 20K each, rheumatism sufferers get a luxury cabin* on a Japanese whaler for a 40-day journey, during which time they get to watch whales being killed, then relieve their aches and pains immersed in still-oozing whale carcasses.
*with optional bunk and porthole.
I read your post carefully then unconsciously switched to skimming just before I hit this part:
Which I read as “with optional port & bunghole”.
I’m not sure which is worse. Although I bet a diet high in blubber just might provoke some GI symptoms in folks unaccustomed to such fare.
CostCo said there was a shortage and wouldn’t give me a 90-day supply 
The co-pay is top tier.
Dang insurance companies.
There’s a now-hilarious passage in Moby Dick (Chapter 87 “A Squeeze of the Hand”) where the whalers hand-process some freshly harvested spermaceti. As the oil was literally named after its resemblance to semen, it now elicits snickers from every student assigned to read the book.
Squeeze! squeeze! squeeze! all the morning long; I squeezed that sperm
I just looked at this again, and I’m thinking that this methodology can actually be dangerous. Whale blubber is slippery; heck, they used to use it for oil lamps. So, let’s say you are sitting on top of a whale with your legs immersed in the slit when, suddenly, you slip and fall right into the slit! Within a moment, you are drowning in blubber! Your muffled crys go unheard, and nothing can avert the tragedy.