I live in an earthquake area and while I had used blocks to temporarily experiment with different heights I needed more stable solution. My bed is solid wood frame, I drilled two holes into each leg and used bolts with nuts to attach extra legs that are 7'' longer than the original ones. The new legs are vertical and beveled 5 degrees at the top so the bottom sits flat on the floor and the beveled top side supports fully the frame. I used similar kind of wood the frame is made from and it looks almost like they are the original legs. Also they can be easily removed and re-bolted again if necessary, a feature I used a few times. See my post "After two and a half months", which I will be updating shortly since I inclined my bed again with better results.
The pictures below should give a better idea.

I like your idea, very neat and stable.

The simplest ideas are always more impressive and yours ticks all the boxes.

Thanks for sharing