I finally found the time today to dig into oldsys-preseed and what I found wasn't pleasant. In fact, I may have solved the mystery why some people trying to install Debian on their NSLU2 cannot connect to the installer via SSH.
oldsys-preseed is used on certain NAS devices to read the network configuration from the original firmware and these values are used to bring up SSH in debian-installer. If you used DHCP in the original firmware to obtain an IP address, debian-installer will do the same. If you have a static IP config, debian-installer will use it. In theory, it's all pretty simple, but users often run into troubles when a static IP config is used (as is the case on the NSLU2). While you only need an IP address, netmask and gateway to bring up SSH (and then you could configure the rest, such as DNS and hostname), debian-installer expects everything at the same time. I've seen many cases where users couldn't connect to the installer because they forgot to configure their DNS in the original firmware or some other values they didn't think were important.
Anyway, right now oldsys-preseed will use a static configuration if the firmware is configured this way even if it's clear that netcfg won't like this configuration because of missing values. The IP address and DNS are always required, but the netmask and gateway can be figured out automatically. I wanted to add a sanity check that would choose DHCP in case the IP address or DNS were not specified.
While working on this sanity check, I noticed two things: First, the "empty" value oldsys-preseed preseeds when there's no netmask or gateway is not accepted by netcfg and leads to an error message (which the user cannot see because SSH is not up yet). I'm pretty sure I tested this scenario when I wrote oldsys-preseed but it clearly doesn't work today and I don't see how it could ever have worked. This is pretty significant because having an empty gateway is a valid configuration (if hopefully not too common). Second, it seems that it's impossible to create a preseed file that tells netcfg that no gateway should be used. If you preseed an empty value, netcfg will simple use .1 as the gateway.
I've fixed the first problem and proposed a patch to put in a special case to allow preseeding of no gateway. The sanity check that chooses DHCP when the network configuration is incomplete is also there now.