For what it's worth, I implemented this a few weeks ago for the exact
same reason, and chose '-n' as well.  So it's intuitive to me.

Here's my patch in case it is at all useful.

Index: du.1
===================================================================
RCS file: /QUIST-CVS/netbsd/src/usr.bin/du/du.1,v
retrieving revision 1.1.1.2
retrieving revision 1.2
diff -u -r1.1.1.2 -r1.2
--- du.1	2001/12/01 19:03:31	1.1.1.2
+++ du.1	2003/02/13 13:59:46	1.2
@@ -43,7 +43,7 @@
 .Nm
 .Op Fl H | Fl L | Fl P
 .Op Fl a | Fl s
-.Op Fl ckmrx
+.Op Fl ckmnrx
 .Op Ar file ...
 .Sh DESCRIPTION
 The
@@ -75,6 +75,11 @@
 .Fl k
 flag is specified, the number displayed is the number of kilobyte
 (1024 bytes) blocks. Partial numbers of blocks are rounded up.
+.It Fl n
+Files and directories with the nodump flag set are treated as if they
+were zero size.  Directories with the nodump flag set are not
+traversed.  This is intended to mimic the behavior of
+.Xr dump 8 .
 .It Fl m
 If the
 .Fl m
Index: du.c
===================================================================
RCS file: /QUIST-CVS/netbsd/src/usr.bin/du/du.c,v
retrieving revision 1.1.1.2
retrieving revision 1.2
diff -u -r1.1.1.2 -r1.2
--- du.c	2001/01/04 23:05:54	1.1.1.2
+++ du.c	2003/02/13 13:59:46	1.2
@@ -76,13 +76,15 @@
 	long blocksize, totalblocks;
 	int ftsoptions, listdirs, listfiles;
 	int Hflag, Lflag, Pflag, aflag, ch, cflag, kmflag, notused, rval, sflag;
+	int nflag;
 	char **save;
 
 	save = argv;
 	Hflag = Lflag = Pflag = aflag = cflag = kmflag = sflag = 0;
+	nflag = 0;
 	totalblocks = 0;
 	ftsoptions = FTS_PHYSICAL;
-	while ((ch = getopt(argc, argv, "HLPackmrsx")) != -1)
+	while ((ch = getopt(argc, argv, "HLPackmnrsx")) != -1)
 		switch (ch) {
 		case 'H':
 			Hflag = 1;
@@ -110,6 +112,9 @@
 			blocksize = 1024 * 1024;
 			kmflag = 1;
 			break; 
+		case 'n':
+			nflag = 1;
+			break;
 		case 'r':
 			break;
 		case 's':
@@ -171,8 +176,16 @@
 	for (rval = 0; (p = fts_read(fts)) != NULL;)
 		switch (p->fts_info) {
 		case FTS_D:			/* Ignore. */
+			/* Ignore anything with nodump set. */
+			if (nflag && (p->fts_statp->st_flags & UF_NODUMP))
+				fts_set(fts, p, FTS_SKIP);
 			break;
+
 		case FTS_DP:
+			/* Treat files with nodump set as zero sized. */
+			if (nflag && (p->fts_statp->st_flags & UF_NODUMP))
+				p->fts_statp->st_blocks = 0;
+
 			p->fts_parent->fts_number += 
 			    p->fts_number += p->fts_statp->st_blocks;
 			if (cflag)
@@ -196,6 +209,9 @@
 			rval = 1;
 			break;
 		default:
+			if (nflag && (p->fts_statp->st_flags & UF_NODUMP))
+				p->fts_statp->st_blocks = 0;
+
 			if (p->fts_statp->st_nlink > 1 && linkchk(p))
 				break;
 			/*