It is mentioned multiple times in the iEEG specs that we can place the electrodes using a CT scan, but is there currently a way to have a CT scan in a BIDS dataset?
Many SEEG centers use post-implantation CT scan to locate the electrodes, not MRI.
There is a proposal to extend BIDS for CT scans, but it is still under development and does not appear to have been touched since Dec 31st 2018:
Can’t find NeuroStars account for the person listed as the moderator to ping them here.
This proposal can give you a framework for how to organize the data. You will still need to tell the validator to ignore the directories created until the proposal is merged into mainline BIDS.