You can run it in Wine or Proton probably, adding the exe to steam and running it with proton worked for me for SourceUnpack but I haven't tried 3420.

Yeah it works, I just installed Source Unpack and added portal.bat as a non-steam game. Then, through steam I checked the box in the "Compatibility" tab to run it through proton-ge.

Side note: borderless windowed mode doesn't seem to work on Linux, as the game seems to just default to Fullscreen, preventing use of Livesplit unless you like staring at your taskbar.

Livesplit works in Linux under wine, or you can compile LivesplitOne. I've gotten both working in Debian 10 and 11. What does not work is the autosplitter functionality, so you'd have to use a hotkey to trigger the splits. If anyone has figured out how to make Livesplit work with autosplitter under Linux, I'd love to know. I haven't had to run a Windows machine in about a decade, but I had to build one last month when I decided to speedrun Portal Reloaded. I spent a good couple weeks trying to get Livesplit to talk to SAR in Linux, no luck here.