1. For your "Format and partition" tasks, just create one partition using the entire disk
2. Download the Bitlocker Drive Prep Tool - http://www.microsoft.com/downloads/details.aspx?FamilyID=320b9aa9-47e8-44f9-b8d0-4d7d6a75add0&displaylang=en
3. Install this on a test workstation and copy the files from the install dir (if i remember correctly it is in Program Files) to all distirubtion points. I have MDT integrated into SCCM in my environment, so i copied the files into the Application Dir of the MDT package.
4. Create a new task in your State Restore Phase with this command line: bdehdcfg.exe -target C: shrink -newdriveletter s: -size 1536 -quiet
5. Working dir for my task looks like this: %deployroot%\Applications\Bitlocker Prep Tool\x86
6. Add a restart computer task after this step
7. Add the "Enable Bitlocker" task after the reboot
This will work for anything that is being started through a PXE boot or boot from media. For a refresh scenario, you will have to add a couple additional tasks:
1. During the State Capture phase you will need to add a task to detect if Bitlocker is configured and set/add a TS variable to use later.
2. During the Install phase, directly after the "Restart to Windows PE" task, add an additional "Format and Partition Disk" task and add a condition so that it will only run if Bitlocker was detected during the State Capture phase
I am not sure if you resolved your issue, but here is what i did to get a specifc drive letter for the second partion: