This is already implemented in the player.

If it does not detect a DHCP server, it assigns an IP address within a certain range. This is a predefined range which (I think?) happens to be in the same range as Windows PCs that fall in the same situation (when they can't find a DHCP server either). I believe the range is something like 169.254.xxx.xxx.

But what you're asking for is to define your own fallback address. I don't think this would get implemented, as it would be terribly nonstandard.

I think I know the reason you want to do this. You want to move the player between two networks: A fixed-IP network at your home and a DHCP-based network at your job. I did this for quite a long time before switching to DHCP at home. Instructions on how to do this can be found here in the FAQ.