Currently the end crystal placement feature is more or less a global toggle. Either they can be placed on any block or just vanilla obsidian and bedrock. What I have done is add variations of obsidian which are functionally the same as vanilla obsidian except that you can't place end crystals on them. They along with vanilla obsidian and bedrock are the only blocks intended to allow end crystals to be placed. All other blocks are not meant to sustain end crystals.