read permissions on specific HKCU path

All,

we'currently developing a GPO that wil lset a registry key within HKCU.
This key has to be protected and we need read-only access for the given user.
I tried to set the permissions by using the security settings within the GPO but HKCU is not available.

Is there a way to restrict the permissions to read-only?
(Btw: Having read only on that specific key is not an issue for the software that will rely on that key as the key normally resides within HKLM. But we have to switch to HKCU)
I know there are default path (like HKCU\SOFTWARE\Policies having readonly per user per default. But this is not an option.

Thanks
René