Today I ended building the main board for this little device. The thing consists of two one-bit registers, connected to common bus via tri-state outputs. The schematic is as this:

As one could see, the memory elements are resettable transparent d-latches, made of four NAND gates each.

Here is how the board looks like:

Also, there is an annotated image saved as pdf file.

The board seem to work. There is other one pending - the board with all needed controls, so as to be able to handle these registers properly and be indeed sure if it's working as intended. As of now, all these hanging wires are a mess.