As a background if someone wants to take a stab at this. The fields are loaded usually by a JLayout using this call:
<?php echo JLayoutHelper::render('joomla.edit.params', $this); ?>
That JLayout just iterates over each fieldset and renders it.
With $this->ignore_fieldsets = array('jmetadata', 'item_associations'); one can exclude some fieldsets from being rendered by that JLayout.
Something similar needs to be done for Hathor for it to support custom fields in the forms.

I was testing my custom fields, then switch to Hathor [Joomla! 3.7RC1).
I can see all my customs fields in [Fields Tab], and also User Custom fields.

@jbreeding Articles > Edit Field:

System information

3.7.0-rc1 (nightly Build)
macOS Sierra, 10.12.4
Firefox 52 (64-bit)

MAMP 4.1.1

• PHP 7.0.15
• MySQLi 5.6.35

System Information

Joomla! 3.7.0-rc1 [ Amani ]
Apache/2.4.20 (Ubuntu)
Version 53.0.2785.143 running on Ubuntu 14.04 (64-bit)

Apache/2.4.20 (Ubuntu)

PHP 7.0.17
MySQLi 5.5.54

It seems good ?

Sorry it does work on users - it doesnt work on content or contacts

Ok, got it.

User => OK
Contact => NOK
Content => NOK

you right for both.

Edit Content:

Edit Contact:

I am on it. Patch will be ready this afternoon.

We will get (here for articles):

Thanks - dont forget that "Fields" is just for fields that are not in a group and you would need a new slider for each group

Set to "closed" on behalf of @infograf768 by The JTracker Application at issues.joomla.org/joomla-cms/15205

Closed as we have a patch #15227

Thanks @infograf768