I am lucky in that I have a model with 128KB of RAM, but the OS does not have any code to store data in the extra RAM except for one of the RAM pages which it uses for USB stuff and mathprint. The calculator can have 64KB mapped (that is 2^16 bytes) at any time, so to access the rest of the flash or RAM, we need to work with ports. If you did manage to add more RAM, the current setup of the ports could allow up to 2097152 bytes of RAM (about 2MB).

I have made a few programs that allow you to store data to the extra RAM and retrieve it. The best version detects how much RAM you have and even assumes there might be future models with a different number of RAM pages other than 2,3, or 8 and allows for more custom naming schemes.

I don't know if anybody has done it, but you could use the eZ80 to replace the z80 processor, I believe, but that wouldn't be overly useful with the current OS and LCD. You would have to rewrite most graphics routines to help offset the LCD bottleneck. For example, don't update the LCD while graphing. Instead, graph everything on a buffer, then update the LCD all at once. Anything not graphics related could see as much as a 50-fold increase in speed.

Also, I cannot help with this as I know very little about actually working with the hardware XD

I still think improving an 86 with flash is the best choice if all you want is more RAM.

Timing is the most difficult obstacle you will face when adding hardware to the data bus of the ez80 CPU. 8-bit Guy once did a hack a z80, if I remember correctly. He got a donation from a friend who build him a timing circuit out of a much more powerful microprocessor than the z80 micro controller. Originally 8-bit guy wanted to use transistors and such. The switching speed of transistors that are easy to solder by hand switch much slower than the TI-x Calculator. When I was throwing together my own z80 motherboard, I thought of ways to read the pins on the data bus and write software to know the extra hardware that I would add to extend the memory, (at slower speeds), since the addresses available could not count higher than the existing 16 pin configuration of the z80. The ez80 has a higher pin count, but the addresses are limited again within the ez80. TI has probably managed the memory addresses well. If you want more RAM then I suggest an HP Calculator with an ARM chip.