Can a Bluetooth wireless connection bridge a USB Host and USB device?
Yes, this is possible.

As you pointed out, you could implement the interface to the USB host using a USB to TTL UART interface.  You would still need a microcontroller in-between to set up the pairing etc. with the other Bluetooth device.  So you can't just connect the UART directly to the TX/RX leads of the Bluetooth module.  Because of the serial interface, you may be limited to as low as 115,200 bps, certainly no higher than 1 Mbps, which is not even as fast as the slower USB 1.1 rate of 1.5 Mbps.

However on the slave side, you will need to implement a USB host to talk to the slave USB interface.  This is not trivial.  There are quite a few chips that implement a hardware USB slave interface, but far fewer that implement a hardware USB host interface.  On top of that, you will have to implement a USB stack. If you want to be able to handle connections to USB hubs, its even more difficult.  Depending on the microcontroller you pick, you might be able to get a library to help with this, but plan on spending a lot of time getting it working.