Below are the changes / hacks I needed to make to make the plugin work with the onestepcheckout.com module:
Add a custom field at position #6 in the OneStepCheckout configuration:

Modify Block/Checkout/LayoutProcessor.php to add the fields at sortOrder position #6:

Small change to the CSS, remove the following from Experius/Postcode/view/frontend/web/css/source/module/_checkout-fields.less

Result:
