I don't believe so. I haven't seen anything on UAs website, and the purser on my last flight with UA had no idea. Last time I landed on a UA flight on CDG there was a GS rep to greet me with a FastTrack pass for immigration. I thanked her but told her that I don't need it thanks and asked about the shower as I had heard about it vaguely in past. She just told me to go to the Sheraton; no pass, etc. I went there and asked at check in and they signed me in, gave me a key, instructions, and a time limit. (I was taking the train onwards to another destination in France and actually not going home for a shower after my flight)

I don't know how easy it would be for someone who is unfamiliar with CDG and who doesn't speak French ie if they want to take the risk of just showing up, but if one is taking the RER into Paris anyways, it really isn't a detour to go and ask.