it's not on our list currently since not too many people have asked for it, but we're not against it.
In general, we want to give developers access to all peripherals and sensors through JavaScript. But since there is often an art to crafting the API that's exposed to JS (since there will be overhead in making each roundtrip across the bridge), we try to ground our APIs in real user use cases rather than just making them before we have a bunch of requests for something with clear goals that we can understand.
One thing that's tricky about HealthKit in particular is that it doesn't have an exact analog on Android (though there are some similar things on some Android phones I believe). We try to make all our APIs work the same on both platforms whenever possible, and it's not totally clear what that means here or what cross platform developers would want.
Let us know more about your use case and that will help shape the way we approach HealthKit! Though it probably isn't something we'll work on in the next few months.