I vote Bitstream. Passthrough is indeed more descriptive, but Bitstream's meaning is universally adopted and understood - it's in my Freesat set-top-box for example. It's a term that means passing through the audio without decoding. Passthrough may be more descriptive however it's not universally used for that concept in that context. I would have to think twice if someone talked to me about audio passthrough in this context. Passthrough, to me, means for example an HDMI signal being available on an AV Amp's output socket as well as being ingested and processed by the amp. It also means an RF signal being daisy-chained between several devices on its way from aerial to TV, it will hopefully not get degrdaded on the way but it may do. I think, in the context we are using it, Bitstreaming means an encoded digital audio signal being sent to a playing device which will decode the signal itself, rather than having the signal decoded before it's sent.
Essentially, the accepted meaning of Passthrough I think is duplicating the input to be sent to two different paths - one of which is consumed by some processing and the other goes onto a daisy-chained device which should be unaware that it is receiving a copy, although the copy may not be completely identical to the original. Bitstreaming means un-decoded or un-converted, i.e. not decoding or converting a single path.