The only way I can think of is to use collision boxes that have their primitive set to "L_NAVCUT", but I'm unsure whether this will help in your situation. I'll give you an example of how I used it, then you can see if it will work for you:
In my mod, I have some market stalls that get added to the town after the player purchases an upgrade. I have my navmesh set up in the empty space as normal. When the upgrade is purchased, I enable the xmarker that contains my stalls. The problem was that the navmesh is still running underneath the stalls, causing the NPC's to try and walk through it. So I added a navcut collision box so that it covers the stall and set its enable parent as the same xmarker.
What it does is as the name implys, it "cuts" out any navmesh it is sitting on top of.
Hope this helps
- Hypno
EDIT: Just realised, this probably won't help you with your platforms. I'm ot too sure of a way to dynamically alter the navmesh during the game...