I'm not so sure you can kill a specific activity that is cached in the background unless it is something that is in a newer OS as you said.
After using B4A for over a year now, I've fixed plenty of problems that came up when my app was cached in the background and it was force-closed either by the user, a task killer or the OS. I've been able to diagnose and fix all of them by simply force-closing my app in the phone's built-in task manager in order to test things.
I would agree that seeing what happens to your app when this happens is a very important part of writing a reliable Android app. It can be a huge source of Null Pointer Exception errors.
One way I found to somewhat reliably have my app's other background activities get killed by the OS was to press Home while on one of my activities and then use the heck out of the phone by running memory-intensive apps such as large games, browsing websites and watching videos, etc. I used this method to track down a NPE problem a few times.