Any built-in media server (Twonky or otherwise) of a NAS is independent of the streaming solution that utilizes the NAS for storage and to pull content. If a NAS has built-in media server, it will operate under it's own environment for purposes of serving media to computers (or compliant devices like game consoles) that are connected to the network - it will do so either thru a browser-based application or a stand-alone application developed by the NAS manufacturer. By media server I mean serving music, video, photos.
If a NAS is used in conjunction with a streaming solution like Sonos and SqueezeCenter, the Sonos and SqueezeCenter applications are themselves the "server", pulling content from the NAS and serving it to the rest of the chain via their respective players or bridges. The Sonos and SqueezeCenter applications do not utilize the built-in media server of the NAS. Instead, it only utilizes the file sharing utility of the NAS by way of the SMB/CIFS file system, which btw all NAS boxes have this feature. For example, in the NAS I'm using I can turn-off/disable the built-in media server but Sonos can still pull and serve music to my audio system.
Album art and metadata that are either automatically written or manually embedded during conversion (ripping) are simply read and served up by either the built-in media server of the NAS or the streaming solution's server application, whichever you're using and, once again, are independent of each other. The upcoming PSA Bridge for the PWD will most likely operate the same way - it will simply read the data and serve it. The ability to pull missing album art or metadata rests either on the built-in media server (if it has one) of the NAS, or on the streaming solution's server application (Sonos, SqueezeCenter, whatever).
The same applies to codec/format support - for instance, the built-in media server of a NAS may not support a particular audio format but the streaming solution's server application may support it, or vice-versa. Therefore, the "desired features or server attributes" you mentioned that you're looking for to match the upcoming PSA Bridge will depend on the quality and comprehensiveness of the Bridge's server application itself and not on the built-in media server of a NAS that you choose later on.
Noise factor and scalability of capacity are other features worth considering when choosing a NAS.
In your case, the no-computer goal to stream audio will largely depend on how PSA ultimately configures the Bridge and its accompanying server application, not on the media server functionality of a NAS. If DLNA and UPnP compliance are the only requisites, then you're good to go. However, if you're going to use a NAS to serve content to other computers or devices in your network then yes you need to consider all the media server features of the NAS.
I believe we had a similar discussion in another thread but I hope this clarifies things a bit for you.