I have found it! The amazing PS3 adaptor does the job. It is a full USB host and will charge any USB standards compliant device under the sun!
Details
After spending weeks going through dozens of generic and not so generic wall to USB power adapters, some of which work with some devices, and some that work with others, the PS3 adaptor worked with everything. I literally threw a dozen different USB devices at it and it charged them all flawlessly. Victory!
Even though it says it is only compatible with PS3 accessories, it appears to be an actual USB host, which intelligently does a USB handshake with the slave device before charging. The slave device can specify how much load it needs in this handshake, up to 500mA as per the USB standard, the host provides the power, and when the slave device is fully charged, it asks the host to stop and no more power is exchanged.
As opposed to the USB Charging Port, which is simply a "dumb" power port with data pins shorted, this adaptor does a full handshake, and therefore anything you can charge with a PC without special drivers, you should be able to charge with this adaptor. This also won't overcharge your device since power isn't just constantly supplied like in a dumb port. Other users have attested to the same.
Testing with the Sony Reader
In my own experience, this clearly shows when charging the Sony Reader. I have previously charged this device using a UCS standard mobile charger. The charging light immediately goes on and there is otherwise no other reaction from the device (the device does correctly charge).
When using the PS3 charger on the other hand, connecting the charger does not result in the charging light to go on. Instead the Reader turns on, boots up, shows on screen: "USB connected", then "USB charging". At this point, after the handshake, the charging light goes on. When fully powered, the screen displays: "USB charging complete". This behaviour is identical to when you plug the device into any other full USB host such as a PC.
Notes & Conclusion
Because the PS3 adaptor is a standard USB 2.0 host, it only gives up to 500mA as per the spec. There are 2 USB sockets in the adaptor and each is rated at 500mA, however those cannot be combined to output 1000mA, unlike the dedicated charging stations.
Therefore charging speed for your devices will be comparable to plugging them into a computer rather than a wall charger, and devices outside of the USB spec that require >500mA (eg. the iPad) will not be charged.
This however is not an issue for me. As noted in the original question, my concern is only with maximum compatibility, not maximum power or charging speed. And to that end, the PS3 adaptor works perfectly :)
Best Answer
The U means universal indeed, but some manufacturers (such as Apple) choose to provide their own proprietary USB cables -- which are not interchangeable -- while others provide generic USB cables which are interchangeable.
All standard USB connectors by pinout.
In picture form: micro USB on the left, then mini USB, then regular full-size B and A USB.