There's no Scala runtime. I think you're either mistaken or using the wrong term. Do you mean the standard library? If so then what's the problem with adding that to your classpath?

I get that some of the Scala constructs (e.g. traits) may be unweildy to use from Java, but I think you're unlikely to have a large and growing surface area for that. On a case-by-case basis I think you could just provide a Java friendly interface from your Scala code.