Short version: rather than trying to secure the empeg (difficult and taxing on its CPU), put it behind a firewall.

Long version:
I'll be worrying about security when I install my carputer. I plan to use it to let me upload music to the empeg while it's still in the car (soon after parking).

Since I don't trust either WEP or WPA for access to my home network, I hacked my WRT54G to be an IPSec endpoint. Now I can VPN from my laptop to the Linksys (I allow others free Internet access, but my home lan is only visible to IPSec clients). The down side is that the Linksys's CPU is so slow that it can only push ~1.5Mbps through an IPSec tunnel (not enough for streaming video from the TiVo to my laptop). I'll soon build another linux box to be a VPN server (likely openvpn).

Also wanting to keep the empeg secure against wardrivers (as I'll have a PC in the trunk with ethernet to the empeg and WiFi), I'll config the carputer to be a firewall too. Then when in range of my home WiFi, it can VPN in (net-to-net) so I can upload new music.

The empeg shouldn't need any modification so no extra CPU or memory usage to slow it down.