I expect it to work fine without steam. Nobody is going to invest in a completely propriety comms protocol. it will probably be usbHID and a usb keyboard descriptor(or whatever the bluetooth equivalent is). Instruct your usb attach code to attach it as a game pad and it will work fine.

However, the configuration utility for it is part of steam, it is a highly configurable controller, so much so that it could be argued much of it's utility is lost without this configurator.