When we deployed Windows 7 we ran into the same problem. Our solution was a bit simpler!
Connecting to root\cimv2\Security\MicrosoftTPM and executing
select * from win32_tpm
will only return a value if the TPM is enabled. This can be added as a condition in your Task Sequence so that your Bitlocker steps only run if the TPM is on.
A note of caution though - this query does not check whether the TPM is activated. It only checks that the TPM is enabled.