- Implemented Quasar's suggestion to use line flag 2048 as a "reserved" flag

that, if set, clears all the non-original-Doom flags on a line. This is
  only for Doom format maps when playing Doom.

SVN r124 (trunk)

docs/rh-log.txt

@@ -1,4 +1,7 @@
 -May 16, 2006
+- Implemented Quasar's suggestion to use line flag 2048 as a "reserved" flag
+  that, if set, clears all the non-original-Doom flags on a line. This is
+  only for Doom format maps when playing Doom.
 - Added support for automatically loading ACS objects (even for Doom-format
   maps). To use it, compile the ACS files as ordinary libraries placed
   between A_START/A_END markers. Then outside the markers, create a lump

src/doomdata.h

@@ -157,6 +157,11 @@ static inline int GET_SPAC (int flags)
 // [RH] In case I feel like it, here it is...
 #define ML_3DMIDTEX_ETERNITY		0x0400
 
+// If this bit is set, then all non-original-Doom bits are cleared when
+// translating the line. Only applies when playing Doom with Doom-format maps.
+// Hexen format maps and the other games are not affected by this.
+#define ML_RESERVED_ETERNITY		0x0800
+
 // [RH] Extra flags for Strife compatibility
 #define ML_TRANSLUCENT_STRIFE		0x1000
 #define ML_RAILING_STRIFE			0x0200

src/p_xlat.cpp

@@ -92,6 +92,13 @@ void P_TranslateLineDef (line_t *ld, maplinedef_t *mld)
 	}
 	else
 	{
+		if (gameinfo.gametype == GAME_Doom)
+		{
+			if (flags & ML_RESERVED_ETERNITY)
+			{
+				flags &= 0x1FF;
+			}
+		}
 		passthrough = (flags & ML_PASSUSE_BOOM);
 	}
 	flags = flags & 0xFFFF01FF;	// Ignore flags unknown to DOOM