chainload EFI from alternate TFTP server

[eterle]

I think I could boot undionly.kpxe, then pick up boot.ipxe to bring up a menu, as described in https://gist.github.com/robinsmidsrod/2234639, and in that menu, include all the desired install options.

So I set up undionly.kpxe, I can see my dhcp client load the file:

101 0.00652 TFTP Read "undionly.kpxe" (octet)
102 0.00198 UDP D=1193 S=45180 LEN=23
103 0.00015 TFTP Ack block 0
104 0.00075 Data block 1 (1468 bytes)
105 0.00017 TFTP Ack block 1
106 0.00012 Data block 2 (1468 bytes)
107 0.00027 TFTP Ack block 2
108 0.00010 TFTP Data block 3 (1468 bytes)
109 0.00016 TFTP Ack block 3
110 0.00010 Data block 4 (1468 bytes)
111 0.00016 TFTP Ack block 4
..
194 0.00012 Data block 46 (1179 bytes)
195 0.00027 TFTP Ack block 46

Then, all I have time to see is "boot failed" and it loads the existing grub from the hard disk.
The network card is Broadcom BCM5719 A1 which is "expected to work" according to http://ipxe.org/appnote/hardware_drivers.
Bootcode 1.47
MBA 17.2.1
EFI 17.4.8

I'm at a loss for how to troubleshoot this. Any help is appreciated.
Thanks.
Darie