So this turned out easier than I expected

dllmain.cpp

#include "pch.h"

#include <winsock.h>
#include <detours.h>

#include <map>

#pragma comment(lib, "user32.lib")
#pragma comment(lib, "ws2_32.lib")

static std::map<HANDLE, char*> HANDLE_TO_POINTER;

HANDLE(WINAPI* Real_WSAAsyncGetHostByName)(HWND hwnd, u_int msg, CONST char* name, char* buf, int bufsize) = WSAAsyncGetHostByName;
BOOL(WINAPI* Real_GetMessageA)(LPMSG msg, HWND hwnd, UINT filterMin, UINT filterMax) = GetMessageA;

BOOL WINAPI Mine_GetMessageA(LPMSG msg, HWND hwnd, UINT filterMin, UINT filterMax) {
    BOOL result = Real_GetMessageA(msg, hwnd, filterMin, filterMax);
    if (result < 0) {
        return result;
    }

    if (msg->message == 0x8001) {
        if (!WSAGETASYNCERROR(msg->lParam)) {
            HOSTENT* host = reinterpret_cast<HOSTENT*>(HANDLE_TO_POINTER[(HANDLE)msg->wParam]);
            if (host->h_addrtype == AF_INET) {
                host->h_length = 4;
            }
        }
    }

    return result;
}

HANDLE WINAPI Mine_WSAAsyncGetHostByName(HWND hwnd, u_int msg, char* name, char* buf, int bufsize) {
    HANDLE handle = Real_WSAAsyncGetHostByName(hwnd, msg, name, buf, bufsize);
    HANDLE_TO_POINTER[handle] = buf;
    return handle;
}

BOOL WINAPI DllMain(HINSTANCE hinst, DWORD dwReason, LPVOID reserved)
{
    if (DetourIsHelperProcess()) {
        return TRUE;
    }

    if (dwReason == DLL_PROCESS_ATTACH) {
        DetourRestoreAfterWith();

        DetourTransactionBegin();
        DetourUpdateThread(GetCurrentThread());
        DetourAttach(&(PVOID&)Real_WSAAsyncGetHostByName, Mine_WSAAsyncGetHostByName);
        DetourAttach(&(PVOID&)Real_GetMessageA, Mine_GetMessageA);
        DetourTransactionCommit();
    }
    else if (dwReason == DLL_PROCESS_DETACH) {
        DetourTransactionBegin();
        DetourUpdateThread(GetCurrentThread());
        DetourDetach(&(PVOID&)Real_WSAAsyncGetHostByName, Mine_WSAAsyncGetHostByName);
        DetourDetach(&(PVOID&)Real_GetMessageA, Mine_GetMessageA);
        DetourTransactionCommit();
    }
    return TRUE;
}