This site already has a "search in thread" feature:
View attachment 4361769
I was just about to do this. Though i will say it has the same drawbacks all thread searches have. And its the major reason why people don't bother with it.
Wording. words three (four?) letters long get dropped when searching, and if you don't use the same words as someone who is discussing the topic it won't find it. I remember searching for the ruck creature, not knowing its name. And the keywords I used were "wrong" and so i didn't get any relevant answers.
Fortunately when i asked about it they responded kindly, but more than once i've asked a question, that I had searched for, but worded differently and found nothing only to be cursed/chewed out for not using the search function.
It only works if you know exactly what your looking for.