Lee have you made one?
I don't have enough experience to immediately understand how you get to the esp8266 without going through a 6551 (as in Dan Werner's design), BUT I have been thinking you could probably shave off some of the cost and complexity of this by stopping after the 6551 (with a TTL level UART) and then having a connector either for an esp8266 module, OR one of these uart ttl to rs232 converters. $7 for 5 of them. Using these also has the advantage of saving you the cost of having to buy the actual connector...
Oh snap. That’s a great idea.You'd have to program the esp8266 firmware to read bytes off the GPIO pins and do some handshaking instead of making calls to serial.read I reckon. 6502 firmware would need entry points matching those of the SSC that hit the 6522 instead of a 6551. I haven't really sat down and thought about it deeply, nor brushed up on what is required to make a card work with PR# and IN# and such.
Think of it as talking to the esp8266 through a bidirectional parallel port instead of a serial port.