does the object returned by getAccel allow C-style structure field calls?

I'm afraid not... you have to use a jsv... method to access JsVar objects. To test this in your branch I believe as long as you enable GitHub actions for your repo, Bangle.js firmware will automatically be built (or attempted - in this case it would fail) and the files are available for you to download.

Actually when I said library I was thinking that rather than building it into the firmware you'd just add a JavaScript library in https://github.com/espruino/BangleApps/tree/master/modules - sorry for the confusion.

Problem with doing it this way is everyone would have to update their firmware before it was available - and if you ever needed to change the values it'd require another firmware update.

I've just added a library for you at https://github.com/espruino/BangleApps/blob/master/modules/wear_detect.js but initial tests show it could do with a bit of work - closing this PR for now.