How can I create a block area that only appears in a Sub Navigation, or on pages within a Sub Navigation?

I'm trying to create Folder-specific block areas similar to how I would create Page-specific block area.

Example Page-specific block area:

<squarespace:block-field id="sidebar-blocks-{collection.id}" columns="12" />

This puts a unique block area on each allowing per-page flexibility, which is great. However, I'd like to create one that display the collection ID of the Folder Overview page (the first page in a Folder).

If you're inside of a Sub Navigation, you can use this all day long:

submenu.block

  {.section collection}
    <h3>{navigationTitle}</h3>
  {.end}

So I'd like to do that same method, but use {title|slugify} and add that to my block area ID.

Here are things I've tried which don't work:

This would be inside of my submenu block:

submenublock.block

<squarespace:block-field id="sidebar-blocks-folder-{.section collection}{title}{.end}" columns="12" />

It appears that Block field cannot render inside of a submenu block, so that's out.

So I tried this directly inside my template code:

<squarespace:block-field id="sidebar-blocks-folder-<squarespace:folder-navigation template="submenublock" />" columns="12" />

But It appears the Folder Navigation cannot execute inside of the block ID, so it renders as a HTML inside of a block field.

So I'm at a loss, does anyone have any ideas?