Is the situation that you've inherited some legacy web app code with JSPs already written the "old way", with Java code embedded? Since you mentioned the JSTL (<c:import>), I assumed modern standards were being used.
If legacy code, then yeah, a refactor just to add some new functionality isn't called for. But in new code (or legacy code that's so bad that not refactoring prevents any forward progress), JSPs and web app should be written using modern constructs and standards.
But, by all that's good, if your app is already using modern standards,
you should not introduce antiquated practices like scriptlets into the JSPs.
In any case, there are no JSP mechanisms to generate a POST request that's not form-generated. You can use the aforementioned java.net package, or a tool such as HttpClient.