Same question, but for the Mac version. There are ways by creating some files on the command-line and relaunching Steam to force it to DL so you can run it with Ren'Py, but it's kind of a pain.
Any chance you'd be willing to team up with somebody that owns an Apple machine and turn a proper signed Mac build for Steam? I haven't tried it, but there's a script called renotize that signs and notarizes Ren'Py Mac binaries. It requires a Mac to run the thing, along with a Developer ID cert from Apple, but the latter is part of the $99 developer program membership, so it's really mostly a matter of finding someone who owns a Mac and convincing them to download and install Xcode, sign in, and maybe download and double-click some certificate bundles if Apple still hasn't gotten that automation right. :-D