たぶんやりたいのはこうだと思うんだけど、違うかな。

DWORD WINAPI proc(LPVOID param) {
typedef void (*fp_t)(void);
fp_t fp = reinterpret_cast<fp_t>(param);

for(int i = 0; i < 10; ++i) {
::Sleep(1000);
fp();
}
return 0;
}

public ref class Form1 : public System::Windows::Forms::Form
{
public:
Form1(void)
{
InitializeComponent();
IntPtr fp = Marshal::GetFunctionPointerForDelegate(gcnew MethodInvoker(this, &CppCli575::Form1::addText));
::CreateThread(NULL, 8192, proc, static_cast<LPVOID>(fp), 0, NULL);
}

void addText() {
if (textBox1->InvokeRequired) {
textBox1->Invoke(gcnew MethodInvoker(this, &CppCli575::Form1::addText));
return;
}
textBox1->Text += "A";
}