Although I can't find explicit documentation for clipboard pointer, it
seems to be possible to modify clibpoard memory without side-effects.

According to MSDN,
http://msdn.microsoft.com/en-us/library/windows/desktop/aa366596%28v=vs.85%29.aspx

"The global and local functions are supported for porting from 16-bit
code, or for maintaining source code compatibility with 16-bit
Windows. Starting with 32-bit Windows, the global and local functions
are implemented as wrapper functions that call the corresponding heap
functions using a handle to the process's default heap."

"Memory objects allocated by GlobalAlloc and LocalAlloc are in private,
committed pages with read/write access that cannot be accessed by other
processes. Memory allocated by using GlobalAlloc with GMEM_DDESHARE is
not actually shared globally as it is in 16-bit Windows. This value has
no effect and is available only for compatibility. "