Does having developed a local (La)TeX system help in graduate admission?
Like all "extracurricular" activities, it will help, but maybe not as much as you would like. I agree with Jeff that it's an "accomplishment that shows a lot of desirable traits", but fundamentally, if the rest of your application is not sound, this is not going to win them over.

I think it is best to think of this as a tie-breaker - if the rest of your academic achievements are as good as the ones of the next best candidate, this is going to be an unique characteristic that will make you stand out and be more memorable. However, if other candidates have better letters, more research experience, or vastly better grades, the fact that you did something very useful in a fundamentally different area is not going to be valued highly enough.