Catalysis of the reaction: 3-hydroxy-3-(4-methylpent-3-en-1-yl)glutaryl-CoA + 4 H+ = 7-methyl-3-oxooct-6-enoyl-CoA + acetate.
3-hydroxy-3-(4-methylpent-3-en-1-yl)glutaryl-CoA acetate-lyase (7-methyl-3-oxooct-6-enoyl-CoA-forming)