What is it like working as a human resource specialist in an Engineer Battalion S1?

Pretty much the same as every other battalion. S1 shops typically split the duties up among the lower enlisted, one person will handle awards, another will handle something else. I've been in every type of unit except Cav and they all work this way. Reach out to your sponsor though, they can give you more details.

I actually did this, once upon a time.
The biggest difference was that there was a ton of manpower in the organization compared to others. My HHC in the engineers was bigger than my whole Civil Affairs Battalion is now.

Same like working on other S1s except you might have better accomodations since you will be stationed in an engineer battalion