After having read sbt-onejar's source code:
packageOptions in oneJar ++=
Seq(sbt.Package.ManifestAttributes(
new java.util.jar.Attributes.Name("One-Jar-Expand") -> "some-file.txt"))
However, I've realized using OneJar with Jetty is quite a problematic beast due to how Jetty loads JSP files—doesn't seem to work with nested JARs—so I'm looking into alternatives now.
EDIT: I've found https://github.com/xerial/sbt-pack and it seems to achieve the same effect as OneJar with the difference of being standard and thus free of problems, much faster, more customizable, and allows defining of convenient custom program entry points.