I could be wrong but I think you'll find on BB10 it doesn't work that way. Silent mode is silent - no alerts at all.
I think the only option so far is to run 10.3+ and use one of the call blocker apps that is able to utilize caller ID data now available with that OS revision, and whitelist the people you want to hear from.
In the 10.3.1 software. If you go to the notifications and the profile you want to alter. You'll notice when setting ringtones for apps etc, that you have an override setting for volume.
Not sure if this is unique to the 10.3.1 version that AT&T recently released. Which has since been made unavailable.
Make sure you backup anything that's important or not synced before you run the autoloader. Also, don't restore settings. Honestly, the best thing you can do is move all your media to an sd card, sync your contacts and calendar, wipe the phone, run the autoloader, and start from scratch. Your call though