Ok, the following dhcpd.conf works for me. Note that pxeboot_ia32.bin
was renamed by me to pxeboot_ia32.amd64.bin to make it different from
the i386 one locally, the name of the file doesn't matter though.

===

option domain-name "vaasje.org";
option domain-name-servers 192.168.0.1;

ddns-update-style none;

subnet 192.168.0.0 netmask 255.255.255.0 {
  option routers 192.168.0.1;
  range 192.168.0.200 192.168.0.254;
}

host opt {
	hardware ethernet 00:0c:76:18:4d:11;
	fixed-address opt.vaasje.org;
	if substring (option vendor-class-identifier, 0, 10) = "PXEClient:" {
	    filename "pxeboot_ia32.amd64.bin";
	} elsif substring (option vendor-class-identifier, 0, 18) = "NetBSD:i386:libsa" {
	    filename "tftp:netbsd.INSTALL.amd64";
	}
}

===


In /etc/inetd.conf, tftp is enabled with:

tftp            dgram   udp     wait    root    /usr/libexec/tftpd      tftpd -l -s /other/netboot


In /other/netboot, there is:

-rw-r--r--  1 root  wheel  7646712 Apr 26 22:56 netbsd.INSTALL.amd64
-r--r--r--  1 root  wheel    41464 Apr 26 20:51 pxeboot_ia32.amd64.bin


That's all you should need.

- Frank