So here is my progress on getting the script that Pants posted to work.
Before you get excited I have not got this to work completely. 969 jumps is my best and thats after messing with this script for 3 or 4 hours last night. Due to the differences in peoples PCs this will never work the same for everyone. So nearly everyone will have to fiddle with the settings to get it to work if it ever does. My advice is to get it setup and run it over and over. If you notice a consistent problem mess with that part of the script or change your latency. (Ive found 851 latency is good for me and I have a fairly high end pc). Do something like watch a movie on another screen while running this over and over. You only have to get lucky once. Anyway on with my progress report.
A few changes had to be made to deal with the frame rate drop and make things a bit easier for testing.
First off download the script and AutoHotKey and load it up.
Edit the script by adding a line at the very top before the "t::" that says "r::Reload" with no quotations. This will allow you to reload the script while in the game instead of having to alt tab to do it.
Now once you have the script edited Save it and load it and have it ready to run.
Now right click your desktop, go to Display Settings and change your resolution to its lowest setting. Mine is 800x600 (I normally run at 2560x1440 which means the game runs at that same resolution). This will solve 99% of the frame rate problems.
Now run FFIX and get Vivi ready. You should be at the point where he has a ! over his head and it says 0 jumps in the left top corner.
Press "t" to start the script, it will immediately pause. Press "ESC" to unpause it and it will run. If Vivi messes up then let him get up and move out. let the script run through the girls text until she says "Sure, Vivi can play" then press "r" to reload the script. This should put you right back at the beginning so you can try again.
Now I have yet to get this to work. I have gotten to 969 jumps with it though. There are a few hangups. At 50 jumps and at 320 jumps. The script tends to mess up there alot. I believe this to be an error in the script rather than the game dropping frames though and im unsure how to fix it.
See the jump transitions do not start at 20, 50, 200, 300. but at 21, 51, 201, and 301. The script is coded for the previous and not the later I believe. I dont understand the authors code enough to fix it myself but maybe someone else can.
Also to note depending on the age of your pc you may have to adjust the "Latency" setting in the script. This is just the wait period between pressing enter for the very first ! to start and Vivi's actual first jump. This will help align all the rest of Vivi's jumps.
It really sucks that this is an achievement as while I did it back in 2000 in the initial release I now have to much pain in my hands to be able to consistently press a button for that time length. The highest ive gotten manually is just under 300. And that caused so much pain I couldnt play the rest of the day. So getting this script to work is the only way Ill be able to accomplish this achievement.
Anyway I hope this helps progress everyone towards this achievement at least a little bit