It's unlikely that there is a backdoor on the TPM itself. The more likely scenario is that given a TPM serial number or EKpub the vendor could furnish a seed in response to a subpoena or warrant -- however, even this is unlikely, as it would make TPM vendors huge targets for hacking. Also TPM vendors make a big deal of how they don't keep TPMs' seeds, and I tend to believe them, because again if they did keep them then they'd be huge targets.