Just to throw this out there, I've said it before but useful info is...useful
Indeed, as confirmed elsewhere, a particular Odin RTC module I found on one board was interchangeable with the 1287/12887 (I tried and it worked. No, I don't recall the model of the Odin RTC module
). Additionally, it was so much of a clone that I was able to hack on an external battery exactly the same as for the Dallas modules.
Houston Tech modules are different. They are not interchangeable with Dallas (as the folks at Maxim say quite clearly in a document I found after a long search) so replacing them isn't quite feasible, but...Houston Tech modules are not filled with epoxy! The lid pops right off them if you tug at it the right way and you have unobstructed access to the battery.
With Dallas RTCs, assuming the coordination and patience is there, I think the hack is the absolute best solution. People concerned about authenticity who don't want to hack it can rest assured knowing they are using the original RTC module if they do...and if they swap it, all authenticity is gone forever! Oh no!
(but really, the hack is relatively simple and works great, allowing you to install a socket to simplify such things as defaulting the CMOS RAM if your system does not provide a jumper)
Maxim does, as mentioned earlier, still sell these things new. No sense fussing with old stock when you can get them right from the manufacturer (the 12887 at least, which did actually work in my PS/2 Model 30 286)