Turns out it wasn't hard to add miniupnpc to the VS2012 project and just have it build with the rest of the code.

ext/miniupnpc/connecthostport.c

@@ -5,6 +5,8 @@
  * This software is subject to the conditions detailed in the
  * LICENCE file provided in this distribution. */
 
+#define _CRT_SECURE_NO_WARNINGS
+
 /* use getaddrinfo() or gethostbyname()
  * uncomment the following line in order to use gethostbyname() */
 #ifdef NO_GETADDRINFO

ext/miniupnpc/minisoap.c

@@ -1,3 +1,4 @@
+#define _CRT_SECURE_NO_WARNINGS
 /* $Id: minisoap.c,v 1.24 2015/10/26 17:05:07 nanard Exp $ */
 /* Project : miniupnp
  * Author : Thomas Bernard
@@ -19,7 +20,13 @@
 #include <sys/socket.h>
 #endif
 #include "minisoap.h"
+#ifdef _WIN32
+#define OS_STRING "Win32"
+#define MINIUPNPC_VERSION_STRING "1.9"
+#define UPNP_VERSION_STRING "UPnP/1.1"
+#else
 #include "miniupnpcstrings.h"
+#endif
 
 /* only for malloc */
 #include <stdlib.h>

ext/miniupnpc/minissdpc.c

@@ -1,3 +1,5 @@
+#define _CRT_SECURE_NO_WARNINGS
+
 /* $Id: minissdpc.c,v 1.30 2015/10/26 17:05:07 nanard Exp $ */
 /* Project : miniupnp
  * Web : http://miniupnp.free.fr/

ext/miniupnpc/miniupnpc.c

@@ -1,3 +1,4 @@
+#define _CRT_SECURE_NO_WARNINGS
 /* $Id: miniupnpc.c,v 1.141 2015/10/26 17:05:07 nanard Exp $ */
 /* vim: tabstop=4 shiftwidth=4 noexpandtab */
 /* Project : miniupnp

ext/miniupnpc/miniwget.c

@@ -1,3 +1,4 @@
+#define _CRT_SECURE_NO_WARNINGS
 /* $Id: miniwget.c,v 1.72 2015/10/26 17:05:08 nanard Exp $ */
 /* Project : miniupnp
  * Website : http://miniupnp.free.fr/
@@ -49,7 +50,13 @@
 #endif /* MIN */
 
 
+#ifdef _WIN32
+#define OS_STRING "Win32"
+#define MINIUPNPC_VERSION_STRING "1.9"
+#define UPNP_VERSION_STRING "UPnP/1.1"
+#else
 #include "miniupnpcstrings.h"
+#endif
 #include "miniwget.h"
 #include "connecthostport.h"
 #include "receivedata.h"

ext/miniupnpc/minixml.c

@@ -1,3 +1,4 @@
+#define _CRT_SECURE_NO_WARNINGS
 /* $Id: minixml.c,v 1.11 2014/02/03 15:54:12 nanard Exp $ */
 /* minixml.c : the minimum size a xml parser can be ! */
 /* Project : miniupnp

ext/miniupnpc/minixmlvalid.c

@@ -1,3 +1,4 @@
+#define _CRT_SECURE_NO_WARNINGS
 /* $Id: minixmlvalid.c,v 1.7 2015/07/15 12:41:15 nanard Exp $ */
 /* MiniUPnP Project
  * http://miniupnp.tuxfamily.org/ or http://miniupnp.free.fr/

ext/miniupnpc/upnpcommands.c

@@ -1,3 +1,4 @@
+#define _CRT_SECURE_NO_WARNINGS
 /* $Id: upnpcommands.c,v 1.46 2015/07/15 12:19:00 nanard Exp $ */
 /* Project : miniupnp
  * Author : Thomas Bernard

ext/miniupnpc/upnpreplyparse.c

@@ -1,3 +1,4 @@
+#define _CRT_SECURE_NO_WARNINGS
 /* $Id: upnpreplyparse.c,v 1.19 2015/07/15 10:29:11 nanard Exp $ */
 /* MiniUPnP project
  * http://miniupnp.free.fr/ or http://miniupnp.tuxfamily.org/