In the US, in the last 50 years, have there been any accidents known to involve a student pilot intentionally practicing stalls in solo flight?
Yes.

I did a search of the NTSB database, limiting my search to final reports with a search term of "practicing stalls." This lead to about 50 reports. I scanned the reports where the aircraft was a typical trainer, such as a Cessna 172 or Piper Cherokee. I found three reports this way that involved a solo student pilot practicing stalls. Interestingly, only one of these seems to be due to a failure to recover from the stall.

12/2/1990 A student pilot experiences engine roughness and eventual failure during stall practice. Cause: failure to apply carb heat during low power settings, leading to carb ice.

3/22/1989 A student pilot fails to recover from a stall, while practicing too low to the ground. A factor involved was the featureless, snow covered terrain that made it difficult to gauge altitude above the surface.

11/5/1982 A student pilot experiences engine failure due to a stuck valve during stall practice.