can a version catalog be externalized without a plugin? or would doing so always require a plugin

What do you mean with externalized? Creating the toml file for publishing or depending on it as consumer?

Depending on it as a consumer. I'm not sure if I'd use a toml file for this either

Still don't get it what you mean: I have this Gradle subproject (as producer) creating the toml file https://github.com/hfhbd/kobol/blob/main/catalog/build.gradle.kts, that is published here https://repo.maven.apache.org/maven2/app/softwork/kobol/catalog/0.2.12, and this code to consume it:

dependencyResolutionManagement.versionCatalogs.register("kobol") {
  from("app.softwork.kobol:catalog:$KOBOL_VERSION")
}

hmm... I imagine if I had that reference a <fn>.versions.toml that wouldn't fully do what I want. Part of what I want is to have the static accessors pregenerated and shipped, assuming that's possible at all. I'm fairly confident that because I use @Vampire’s hack to allow static accessors in buildSrc that anytime I update this file my configuration-cache is being completely invalidated which currently doubles my CI build time until I merge