I actually purchase one of these just because I was very curious how he designed this. I only purchased the DS1486 replacement, and he was willing to sell that to me for $70. This is still very expensive, but I had to know what he did.
The central part of the design consists of a DS1384 RTC controller; this part is identical to the DS1486 without an SRAM. The DS1384 sits underneath the battery, and an SRAM is on the bottom side of the board. Once you add the SRAM to the DS1384, you have something that is identical to a DS1486. I suspect this is exactly what is inside the encapsulated DS1486 - a DS1384, an SRAM, and a couple of coincell batteries. The DS1384 actually has inputs for TWO backup batteries if desired.
I must admit it's a fairly clever design, although the layout seems less than optimal. The bad part of the design is that the DS1384 is obsolete just like the DS1486, so if you buy any of those parts you will have to buy off of Alibaba, Ebay, or a broker somewhere so the quality is questionable. However, the DS1384 does not have an internal battery like a DS1486 does so it is arguably less risky to buy these parts. But I worry that the mileage on these parts may vary wildly.
After paying $70 for one of these from Ebay I have been busy trying to design my own DS1486 replacement. I actually have a design complete, both the schematic and PCB layout are done. What I did was use an available DS1746 as a sub for the DS1486, but I re-map the regs from lower memory to upper memory that the DS1486 requires. I do this by inverting address bits A4-A16 and then using a GAL16V8 to remap A0-A3.
I think this will work, but there are some differences between the 2 register banks if you look at the datasheets. For example, the enable oscillator (EOSC) bit is in a different spot...but I doubt if the TDS sets/clears it. I won't know if it works until I actually get my boards back and test it.