newer:
now using kernel patch deny_lseek_past_EOF_when_O_APPEND.patch
with this bash patch, testing to see how 'dd' reacts... better:
$ rm /tmp/out;echo abcdefghijklm | dd bs=5 seek=8 oflag=seek_bytes > /tmp/out ; stat --format=%s /tmp/out
dd: 'standard output': cannot seek: Illegal seek
0+0 records in
0+0 records out
0 bytes copied, 0.000102177 s, 0.0 kB/s
Actually written to disk: '0' bytes (-nan%).
0


new:
ok this is bad:
because it makes this fail:
$ rm /tmp/out;echo abcdefghijklm | dd bs=5 seek=8 oflag=seek_bytes > /tmp/out ; stat --format=%s /tmp/out
2+1 records in
2+1 records out
14 bytes copied, 0.0001584 s, 88.4 kB/s
Actually written to disk: '14' bytes (100.00%). 
14

the normal output should be(ie. file size 22 bytes not 14):
$ rm /tmp/out;echo abcdefghijklm | dd bs=5 seek=8 oflag=seek_bytes > /tmp/out ; stat --format=%s /tmp/out
2+1 records in
2+1 records out
14 bytes copied, 0.000112444 s, 125 kB/s
Actually written to disk: '14' bytes (100.00%). 
22

ok so because it's append only, dd cann't seek past the EOF, and thus lseek won't do it's job of skipping past EOF and the subsequent 'write' won't make those \0-es pop up in that gap! ie. dd assumes outfile is never opened in O_APPEND mode.

meanwhile this will always work(the redirect became of=):
$ rm /tmp/out;echo abcdefghijklm | dd bs=5 seek=8 oflag=seek_bytes of=/tmp/out ; stat --format=%s /tmp/out
2+1 records in
2+1 records out
14 bytes copied, 0.000137867 s, 102 kB/s
Actually written to disk: '14' bytes (100.00%).
22


old info:
https://github.com/libcheck/check/issues/188

diff --git a/make_cmd.c b/make_cmd.c
index ecbbfd6e..5db799f2 100644
--- a/make_cmd.c
+++ b/make_cmd.c
@@ -700,7 +700,7 @@ make_redirection (source, instruction, dest_and_filename, flags)
     case r_output_direction:		/* >foo */
     case r_output_force:		/* >| foo */
     case r_err_and_out:			/* &>filename */
-      temp->flags = O_TRUNC | O_WRONLY | O_CREAT;
+      temp->flags = O_APPEND | O_TRUNC | O_WRONLY | O_CREAT;
       break;
 
     case r_appending_to:		/* >>foo */