If you could show us the code you have been using we could take a look at why the beforedrawcell is not being triggered in the TMyPlanner. Other than that, you are using the right approach for drawing customized planner elements.
If your willing to share the code, please send it to use so we can investigate this here. We can then search for a proper solution for this issue. You can send the code sample to support@tmssoftware.com with a link to this forum post.