I can confirm that the Goldberg Emulator project successfully fixes the problem with GOG's botched LAN support for Wreckfest.

I packaged up the fix in an easier to deploy mechanism suitable for LAN events, available here with instructions: github.com/juj/Wreckfest_GOG_offline_lan_mode_patch

The above patch enables offline LAN mode play for Wreckfest that was installed via GOG's offline installers (no GOG Galaxy). Verified that no Internet connection is required so true offline LAN mode works, although remember to enable Windows firewall in the popup prompt - I had to restart the game after doing so.