Odd that they couldn't call mlock using PInvoke; it sounds like the PInvoke export metadata wasn't there rather than C actually being required.
On Windows even for really obscure FNs you can almost always PInvoke if you know the offsets, and if you really want to be evil you can traverse the PEB. There isn't much in low level terms that is beyond the reach of C# since you can manipulate memory directly. I've also accessed hidden COM interfaces by traversing V-Tables using similar direct memory techniques as you would in C.
On Windows even for really obscure FNs you can almost always PInvoke if you know the offsets, and if you really want to be evil you can traverse the PEB. There isn't much in low level terms that is beyond the reach of C# since you can manipulate memory directly. I've also accessed hidden COM interfaces by traversing V-Tables using similar direct memory techniques as you would in C.