I have done this with my Witty and it works just fine!
Flashed my ESP-07 with a NodeMCU "cloud build", using esptool with absolutely no problems. It was one of the reasons I got a Witty board, actually, and it seems to do just what I wanted.
Connect TX , RX and GND like you said.
I have also connected GPIO0, you
have to do this.
For good measure I've also connected GPIO15, it may not be absolutely necessary but... note that the Witty board does some clever stuff so you don't
have to hold the flash button for flashing, it pulls the voltages low correctly for you. I seem to remember that this has something to do with some special signal (GPIO15), I'm not too sure - perhaps someone more knowledgeable can comment.
RST is optional but I've connected mine as it's handy to have a button when connecting to ESPlorer.
Remember to run the chip being flashed on 3.3V not 5. It may(will!??) blow otherwise, unless it's got a built-in 3.3V regulator.
(Funnily enough I was a bit worried about RX being at 5V from the daughterboard, so I put in a voltage divider to drop it a bit. Guess what? I wouldn't work like that! Strange. Maybe it would have with a logic level converter, I dunno. However, once I connected RX-RX directly it worked just fine. The Witty top-module is connected like that, anyway)
I'm going to try it on an ESP-05 soon, I'll let you know how that went, later.
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PS.
1) I no longer think that connecting GPIO15 is necessary (at least not on ESP-07). I had it connected wrong, to GPIO5 (doh) and it still worked fine.
2) I was unable to flash either ESP-01 or even register the ESP-05 with this setup. I don't know why. Maybe it's the setup, maybe it's me, maybe the chips are bad. I'll wait until I get one of those USB adaptors I've ordered for the ESP-01 and then try again.
Apart from that I'm very pleased - I will mainly be flashing ESP-07s because I need the antenna. I program/test the code on the Witty, remove the top module, plug in the one to be programmed and voila! Perfect.
I have also made a "breakout-breakout" board for my Witty, simply a "sandwich-filling" veroboard between the top and bottom modules, which allows me to test the pins properly, just like a proper devkit board. I'll probably end up removing the stupid LED and the LDR.
I'd actually written up a review when I first got it,
here (it was my second ESP8266 module then), I'll put up some pics of my program-flashing setup and the "breakout-breakout" board if anyone is interested.
But my overall impression is that the unit runs just about perfectly for what I want.