OK following the edit/replace rules I added an updated version of the feat based on the code from Durable and improved Initiative. Details below and .user file attached.
Initially it stated there were issues with the feat but it worked fine.
ftThirsBat > ftThirstB
eval scripts: #traitmodify[trSurges,trtBonus,1,""] Timing: before Calc trtFinal
eval scripts: #traitmodify[trInit,trtFeat,3,""] Timing: before derived trtFinal
1. Launch the Editor and create a NEW data file.
2. Go to the appropriate tab (e.g. Race).
3. In the lower left, click the button entitled "New (Copy)".
4. From the list presented, select the thing you want to make changes on. While you're here, make sure you write down the unique id of that thing - which is shown next to its name.
5. A new record is created, so give it a new unique id and tweak the name (e.g. change "ftThirsBat" to "ftThirstB").
6. Go to the "Replaces Id" field and enter the unique id of the original thing you copied (ftThirsBat). Doing this tells Hero Lab to use your new thing anywhere that the original thing is referenced. For example, if you replace a standard language, all races that automatically grant that language will now grant your replacement instead.
7. Save your thing.
8. Save your new content in a separate file from the generated content.
9. Click the "Test Now" button.
Not a direct code edit as that would be wiped by the next update but a decent workaround.