Ketogenesis pathway maps show acetoacetyl-CoA converted to HMG-CoA and only then to the first ketone body acetoacetate. Why this detour instead of directly hydrolyzing off the CoA to get there in a single step? Is it possible that some ketone bodies are made by direct conversion? The enzyme acetoacetyl-CoA hydrolase to do this job exists and can be found in rats.
After further checking, I found that HMG-CoA synthase rather than HMG-CoA lyase is the enzyme which is regulated. This enzyme also plays a vital role in cholesterol synthesis. HMG so produced can either form mevalonate, which leads to cholesterol synthesis, or form acetoacetate. Therefore if acetoacetyl CoA were directly hydrolyzed, it would short circuit the cholesterol pathway.
The enzyme can be regulated by succinylation. Its synthesis can be regulated by several transcription factors. Evidence suggests that hormones play a role in each of these processes. References: Harris, et al., Journal of Investigative Dermatology 114(1) 83-87 (2001); F.G. Haggart, Biochem. J. Mar. 5, 338 (pt. 3) 569-582 (1999); Quant, P.A. et al. Eur. J. Biochem. 187, 169-174 (1987)
Most Biochemistry texts today list HMG-CoA as the source of acetoacetate with no mention of direct hydrolysis of acetoacetyl-CoA. This suggests that if direct hydrolysis of acetoacetyl-CoA does occur, it probably plays a very minor role.