After considerable research, I have arrived at what I believe is the simplest, and cheapest, substitute for the now banned mercury-oxide batteries used by the Topcon RE Super/Super D metering system. I do not claim to be the originator of this idea (I ran across it several different places on the Internet), but I have determined it's suitability for use in Topcon cameras. What follows is documentation of my method.

Most battery manufacturers make hearing-aid batteries that use Zinc-air chemistry. And while there are not 'drop-in' replacements for the PX13 or PX625 mercury cells, these cells can indeed be used.

Advantages--
1. Like the mercury cells, the zinc-air cells have a flat discharge curve, meaning they have a constant voltage output throughout their life, then die suddenly.
2. They are also cheap when compared to other solutions.

Disadvantages
1. The output voltage of zinc-air cells is about 1.4 volts, 0.05 volts higher than the original mercury cells. My calculations show that this voltage difference will result in half an F-stop under-exposure in conditions of strong light (EV = 15). But this error becomes less as the ambient light level decreases.
2. Zinc-air cells have a shorter life than other cells-between three and six months, depending on temperature and relative humidity.
3. Zinc-air cells are activated by removing a sealing tab and require some minutes (up to 30 minutes) for the voltage to stabilize.

For the type of photography I do, I find the advantages greatly exceed the disadvantages.

The paragraphs below show the technique I use for installation of the zinc-air batteries. But before a battery is installed, carefully check the battery well to make sure it is clean and free from corrosion. If corrosion is present, do not scrape the metal to remove it. Rather, clean the well using a cotton swab dipped in vinegar. The acetic acid in the vinegar will neutralize and dissolve the corrosion; then clean the well with another cotton swab dipped in distilled water and let the moisture evaporate.

Material
Zinc-Air Cell     Purchase a type 675 Zinc-air cell. I buy mine from a local drug store and find the 'house brand' works well. These cells have a shelf life of several years so I get the package with six cells.

O-Ring    Buy No. 9 O-rings from a hardware store.
Approximate size: 5/8" (15.9mm) OD,  7/16" (11.1mm) ID, 3/32' (2.38MM) thick.

Washer   Also needed is a No. 10 flat washer (also available from a hardware store). Approximate size: 3/8" (9.5 mm) diameter, 0.04" (1 mm) thick.

Insert a zinc-air cell in the center of the O-ring.

Remove the tab from the cell and insert the cell/O-ring assembly into the battery well. Make sure the + symbol on the cell is showing.

On many of my cameras, the battery cap does not make good contact with the + terminal of the cell. Therefore I place a No. 10 flat washer in the center of the cell.

Install the battery cap. Use a coin to screw in the cap since it may require a little extra torque to force the O-ring into the battery well. Before installing the cap, make sure the two little holes are open and clean--zinc-air batteries must 'breathe' to work properly.

Wait about 30 minutes for the battery voltage to stabilize before making exposure measurements. It is good practice to turn off the exposure meter when not in use. But the life of zinc-air cells is more dependent on time duration, rather than energy used, so forgetting to do this should not deplete much of the cell's capacity. Note: the 30 minute stabilization time is only necessary when the tab is first removed--no need to wait when switching the meter from OFF to ON.

One disadvantage of these batteries is the wait for the voltage to stabilize. If a battery goes dead in the middle of a photographic session, the wait for a new battery to stabilize can be inconvenient. One solution is to pull the tab off a new battery before beginning, so that it will be ready if the old one goes dead. If a replacement is not necessary, simply re-affix the tab to the spare battery and it will revert to it's dormant state with almost full capacity. When, in the future, the tab is removed, you must again wait for the voltage to stabilize.