I use the monitor arm over bed, with a wireless keyboard and trackball. It works and I can recommend it, but:

1. There is limited adjustment potential.

2. The keyboard is not positioned correctly in full supination and my typing error rate goes way up. A fully split design may help this.

3. It's not a good posture for focusing, although it's great for calm viewing.

For focusing I'd go for floor sitting with a lap desk to hold wireless peripherals, and anything that can elevate the screen. This method allows postural adjustment with deep stretches over the course of the day.