OOps It seems I read that question too quick.

Here is some background info:

For native types like structs, classes, enums, and unions, the C++/CLI
compiler generates managed wrapper types as soon as these are used in
managed code. However, these wrapper types are more primitive as one might
think. For structs, classes, and unions, they are value types without any
fields. Only the native size of the struct, class, or union is specified in
the metadata. For enums, a managed enum is created, however, it only
contains the standard field to store the value.

Here is the solution that I would suggest:

To see the enum values on both sides, you should define two enums:

In your assembly a public managed enum:
public enum class E
{ a, b, c };

In the header that is included in C++ projects, an equivalent native enum:
#ifndef _MANAGED
enum E
{ a, b, c };
#else
#using "yourAssembly.dll"
#endif

If the header is included in a file compiled with /clr:* or, the managed
enum will be visible, because
   #using "yourAssembly.dll"
will be compiled with your code.
If the header is included in a file complied without /clr:*, the native enum
will be defined.