Just replying in case anyone comes across this I WAS able to get the BLtouch working on the original Melzi board! Long story short, the diagrams listing the Melzi expansion pins do not apply to the board in the Duplicator i3 they have been rearranged from the standard Melzi configuration!
The pin listed as A4 is actually D27! Took forever to figure that out but I believe what this pin goes to is the beeper/speaker in the control unit so if you use jumper pins instead of the standard LCD plug it comes with you can get the BLTouch working without losing your LCD!
I got this working on Marlin firmware 1.18 I believe. In order to do so you just wire the BlTouch the way listed for the Melzi board in the link peps1 provided (https://www.antclabs.com/wiring3) but make sure to attach it to the pin on the board labeled A4, the position may not be the same as diagrams for other Melzi boards so make sure to find the label on your board.
Once that is hooked up you can enable it in the Marlin firmware following the standard guides but make sure you have this line:
#define SERVO0_PIN 27 //BLTouch orange wire
That will tell Marlin to use pin D27 (A4 on the i3 Melzi board) to control the servo for deploying/retracting the pin.
Important note! In order to enable the auto bed leveling in Marlin the firmware becomes too large for the Melzi board unfortunately, so to get around that you can disable the SD card support, which takes up a huuuge amount of space in Marlin.
To do so, find this line and comment it out with two forward slashes in front of it like this:
That should save enough room to get auto bed leveling working. Note that due to size constraints you will only be able to use Bilinear (#define AUTO_BED_LEVELING_BILINEAR) leveling and not the newer unified bed leveling.
I can confirm all the typical BLTouch functions are working as expected (self test, deploying, undeploying, alarm, etc)! The leveling mesh is created properly and the initial layer has the Z adjustments applied (can watch the z axis motors spinning on first layer)!
Hope that helps anyone who finds this post!