Ah, someone has already thought about this. I was going to write something like this next. 
by default the joystick will give the status of all the directional buttons and the buttons A and C as trigger 1 and 2.
This is incorrect... It should be "buttons B and C"
I was thinking about this as well and I must say that I really don't like the fact that the pin needs to be inverted for compatibility mode. Adding a chip inside usually means that you need to go to shop. 
To be realistic: As long as there is no software for more than 2-button joystics I think that making this easy mod that requires only soldering iron is better option for common users... expetially when the needed connectors + chip here costs more than the controller it self. 
If I read the description of the Mega Drive adapter correctly, then you should be able to plug in a 4-button Mega Drive controller unmodified into an MSX, and read out the 4 directions + B button as 1st firebutton by normal MSX software. Custom software should be able to read button A too in that situation. This because pin 8 is normally lowered to enable readout of an Atari-style joystick (that uses pin 8 as ground). Both Trigger1 & Trigger2 pins are normally input, with a pull-up resistor (so normally goes high unless connected to Gnd or pin 8), but can also be turned low (which would enable readout of the other fire button on an unmodified Mega Drive controller). 'Start/C' signal of the Mega Drive controller would be shorted to Gnd, which shouldn't be a problem if it's a 'pull-low-to-activate' type signal.
If you're happy with 2 buttons as used by normal MSX software, it's probably easier to just re-wire the controller inside (similar as in NYYRIKKI's pix above), since you don't need an adapter in that case. Just did that myself with a small (2 firebutton) Sega-branded joypad - works fine, biggest issue was needing a replacement cable because required pin(s) weren't wired in the original plug. If you use MSX pin 8 (output) as common ground & include a series resistor of around 220~330 Ohm in the 2nd trigger signal (pin 7), then you get a controller that's Arcade Turbo equivalent, which you can safely plug into other systems like Atari/C64/... as well (with 1 firebutton available on those other systems).
The above Mega Drive adapter seems more useful if you want to keep your controllers in original state (or plan to wear them out in quick succession 
), or if you have a use for the 3rd, 4th etc. button.
I ordered 2 pads on 27th may, i got them yesterday. So you have to be a little patient.
Today i did the mod. It's very easy to do and the pads work perfectly!
Thanks NYYRIKKI!!!
I ordered some on 17th june, and they haven't shipped yet. Just sent them a message... hope they'll ship soon. Thought someone considering ordering them might want to know. I think waiting for a month is more than being "a little patient", it's more like "quite a bit patient" .
