You could create this as a custom ACF block called preamble or similar. When registering the ACF block, within its block.json file, you can put “multiple”: false in the supports section to limit it to only being used once.
Additionally, you can set ‘jsx’ to true to make the block reliant on InnerBlocks. InnerBlocks work like native blocks where you edit like normal, not via ACF fields. I would personally also limit the allowedBlocks for InnerBlocks to just the core/paragraph block, as well.
So your block.json would look something like this:
{
"name": "preamble",
"title": "Preamble",
"description": "A styled post preamble paragraph.",
"category": "formatting",
"acf": {
"mode": "preview",
"renderTemplate": "blocks/preamble/preamble.php"
},
"supports": {
"multiple": false,
"jsx": true
}
}
Then your preamble block code would include something like:
<?php $allowedBlocks = array(
'core/paragraph'
); ?>
<InnerBlocks allowedBlocks="<?php echo esc_attr(wp_json_encode($allowedBlocks)); ?>" />
It wouldn’t force it to be the first block, but you could probably do that utilizing something like block templates.
