There have been reports of some people mentioning that there are showers in the Air China PEK T3E Business lounge, but I doubt it from past visits. However, the Star Alliance lounge finder tool reports that there are showers (http://www.staralliance.com/loungeFi...paidClassCode=) - but it's sometimes wrong. On the other hand, the Premier Lounge supposably has showers too (http://www.plaza-asia.com/index.php/locations/beijing) and they have a email for you to check with.

Note that as a Star Alliance Gold member, you can enter the contract lounge of the airline that you're flying with (in this case SQ, so the Premier Lounge) and any *G lounge at PEK (the Air China lounge).

In Shanghai, I think the only lounge there is the Shanghai Airlines lounge (which is no longer a member of Star Alliance, although airlines operating to PVG T2 still have a contract with them).