Update: my local stereo guy got the manual to the Dension box in question, and it talks about Bluetooth hands-free calling, but says nothing about A2DP music streaming. Most likely, it's not supported at all.

Now I'm checking out the much pricier FISCON Basic ($400 or thereabouts), which appears to replace my factory Bluetooth. This is perhaps more promising, since the Bluetooth gizmo has the additional benefit of having access to the steering-wheel buttons and the display between the gauges. The web page is surprisingly lacking in technical details. I've asked the rep to send me the installation and user manuals. We'll see.

Further edit: Various forum posts have complained that these Bluetooth replacements, while they speak A2DP, will only give monaural audio for the car. Grrr.