There are two (or more) methods of debugging early boot stage on a machine before the TCP-IP system is available:
In order of usefulness (to best of my knowledge):
1) Setting a serial port as the default (boot) console, and having the second machine listening on the serial line.
2) Using a USB<>USB debug cable. This is a special hardware device, costs about $80.00 CDN last time I looked at it. It requires a software application running on the second machine, and requires that the booting machine have a built-in "Debug Port" functionality in the USB EHCI controller. (The application is open source as I recall -- don't quote me)
3) Debug via Firewire1394 -- I've not looked at this too much, but the required stuff is part of the kernel. On the second machine, you need an application running (it's open-source as I recall -- don't quote me).
I don't have specific details of how to go about
actually implementing these methods; I'm just aware that they exist. From what I have read, the 1394 method is the most effective, and potentially the cheapest, as it doesn't AFAIK require particular hardware, such as the USB Debug Device.
EDIT - I just re-read your post and noted that one of the machines is 'remote' -- so depending on 'how remote it is' maybe these ideas are not ideal... My apology if these do not help in your circumstance
Good luck,
Sasha