I'm rolling out JET SP8 onto PCs via a startup script, and I wanted to force Sandbox Mode via Group Policy. Searching Gooogle for sandboxmode end.policy to see what somebody else might've already written, I came up with only one (1) result, from the MSAPPS-L@LIST.EMWAC.CZ mailing list.
I slashed up the template that Miroslav Pragl posted, and came up with Access-SandBox_SystemDB.adm. Hopefully this will be helpful for somebody-- it's doing that I need. Be sure to uncheck the "Only show policy settings that can be fully managed" checkbox when using this administrative template, since it's not a "fully managed" setting.