%03f should work as one would expect.

Reference: /n/atom/patch/0ffmt
Date: Mon Mar  3 03:34:11 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libc/fmt/fltfmt.c	Mon Mar  3 03:33:25 2014
+++ /sys/src/libc/fmt/fltfmt.c	Mon Mar  3 03:33:26 2014
@@ -303,7 +303,7 @@
 	 * which is 341 currently.
 	 */	
 	xdtoa(fmt, s, f);
-	fmt->flags &= FmtWidth|FmtLeft;
+	fmt->flags &= FmtZero|FmtWidth|FmtLeft;
 	_fmtcpy(fmt, s, strlen(s), strlen(s));
 	return 0;
 }
--- /sys/src/libc/fmt/dofmt.c	Mon Mar  3 03:33:28 2014
+++ /sys/src/libc/fmt/dofmt.c	Mon Mar  3 03:33:29 2014
@@ -87,13 +87,14 @@
 int
 _fmtpad(Fmt *f, int n)
 {
-	char *t, *s;
+	char *t, *s, c;
 	int i;
 
 	t = f->to;
 	s = f->stop;
+	c = f->flags&FmtZero? '0': ' ';
 	for(i = 0; i < n; i++)
-		FMTCHAR(f, t, s, ' ');
+		FMTCHAR(f, t, s, c);
 	f->nfmt += t - (char *)f->to;
 	f->to = t;
 	return 0;
@@ -102,13 +103,14 @@
 int
 _rfmtpad(Fmt *f, int n)
 {
-	Rune *t, *s;
+	Rune *t, *s, r;
 	int i;
 
 	t = f->to;
 	s = f->stop;
+	r = f->flags&FmtZero? '0': ' ';
 	for(i = 0; i < n; i++)
-		FMTRCHAR(f, t, s, ' ');
+		FMTRCHAR(f, t, s, r);
 	f->nfmt += t - (Rune *)f->to;
 	f->to = t;
 	return 0;