Enable Java editing in String literal with delimiters

I would like to enable Java editing within specific delimiters inside a String literal:

int hour = 5;
String stuff = "It is ${hour} o'clock";

I want text between the "${" and "}" to be editable and highlighted as Java (with completion etc.)  Is there an API I can implement to enable this?

Note I've looked at language injection, but that doesn't seem to support delimiters; it appears to be all or nothing.

Thanks.