I have not had a chance to finish up the prototype yet, I have spent the last week cleaning up an IBM XT clone and repairing an IBM AT (5170) for a local business. during my lunch breaks at work, I designed a simple PCB for a cleaner version of my conventional memory expansion card. a couple of PDFs of the board can be found below, but beware I still need to double check everything on them. On the PCB, I decided to rely on the SRAMs ability to go in to high impedance and omitted the 74LS245. 

the SRAM chip used is an AS7C4096A-12JCN in an SOJ-400 package while the other three chips (74LS08, 74LS32, and 74LS139) are all SOL.

http://content.thetechnickel.com/misc/tandy-1000a/expansion/conventional-only/pcb-pdf/512KiB-RAM-Expansion-bottom.pdf

http://content.thetechnickel.com/misc/tandy-1000a/expansion/conventional-only/pcb-pdf/512KiB-RAM-Expansion-combined.pdf

http://content.thetechnickel.com/misc/tandy-1000a/expansion/conventional-only/pcb-pdf/512KiB-RAM-Expansion-silkscreen.pdf

http://content.thetechnickel.com/misc/tandy-1000a/expansion/conventional-only/pcb-pdf/512KiB-RAM-Expansion-top.pdf

quick note: use these PDFs at your own risk, they still need to be double checked for errors (well, tripple checked as I already double checked them).