This is already possible. Under "SET LAUNCH OPTIONS" in the game's properties you can edit the command used to invoke the game. Add in %command% to represent the game's launch command. See https://support.steampowered.com/kb_article.php?ref=6316-GJKC-7437 for details on configuring games to launch using primusrun (optirun is not recommended).
Ok, thanks for the hint.
Maybe you could document this feature whithin the steam gui, since the dialogue currently suggests that you can only edit the launch parameters. (A simple explanation that %command% is a valid placeholder would already help a lot.)
Nothing extracted yet.
A nice feature for future updates would be to implement custom launch commands for games, in their specific settings.
This would make it easier for advanced users to use workarounds for linux-specific problems like "optirun", or "pasuspender".