@legacy: you still looking for an off-the-shelf schematics for your Dallas NVRAMs replacement, don't you?

A professional solution which will guarantee your data will not be lost when randomly switching the power on/off at your 68k system is not that simple. You have to provide a lot of effort to get something 100% reliable.
You need for example, a 1.25V ref voltage, 3.3V opamp/comparator, 3.3V logic, to be well below 5V and to have enough headroom for the activities, and create a simple FSM which will monitor the rise/fall of the 5V rail, and manipulate the /CS and /WE, /WR signals of your SRAMs accordingly.
All must be fast enough to provide the all necessary stuff during the power-off.
The power off with your 68k system may last <1ms easily (based on the shot your board takes 1.5A @5V).
Have a look with your o'scope how long does it take the Vcc dropping from 5V to 4.75V when you switch the stuff off. During this time you have to do "something" with the SRAM's signals.
The 3V backup is easy, 2 diodes are ok. You cannot power the above logic from the 3V backup, however, as it takes several mAmps usually (3V CR2032 is 150mAH, you want 5 years).
PS: to be more constructive - I would start wit a small "dev board" with 1.25V ref (easy, ie LM385-1.2), small 8pin microcontroller, something like PIC12F or 16F which run from 2V Vcc up (optionally with built-in comparators), a low power dual comparator (you may find some with 2-3V Vcc sure), and 2-3 logic level fets (not sure if N or Pmosfets at this stage) for gating the 2-3 SRAM's signals (like the /CS, /WR, /WE..).
Add some decoupling., few resistors. With this setup you may start to elaborate something reliable. You would definitely need an DSO handy.