Since you are creating a new key, shouldn't you just check the parent's permissions once?
Edit: I am unsure if there are managed ways, but you could try CheckAccess() in Stdprov.dll: http://msdn.microsoft.com/en-us/library/aa384911%28VS.85%29.aspx
Edit2: Have you tried http://msdn.microsoft.com/en-us/library/1w66447a.aspx ?
Edit3:
26) Access checks in .NET
In part 2, we went through performing access checks using the Win32 AccessCheck API. Unfortunately, there doesn't seem to be an equivalent managed function that can perform the task. It's not recommended for you perform an access check in .NET. Instead, you should make use of role-based security to perform an access check for you (This is what ReadSD does. Before ReadSD writes a security descriptor, it needs to check if you are allowed to alter the security descriptor. It does this by reading the security descriptor and calling GenericPrincipal.IsInRole to check for group membership). This only works if your objects are designed for role-based security. It does not work with objects secured by security descriptors.
If you need to perform an access check on an object with a security descriptor (Registry key in our case), you wouldn't use AccessCheck to do so (even in Win32). The proper method is to open up the registry key, and if the security descriptor denies access, you will get an "access denied" exception.
In simple access checks, you may be able to perform the access check yourself with the help of an imperative role-based security (fig. 38):