hi @nerdifand sorry about the delay, was playing around here and got to what you are looking for.
So basically I created a Web Template that will be responsible for retrieving the subgrid data - my example I am fetching the account records and displaying the results in a table:
{% fetchxml accountFetch %}
<fetch version='1.0' output-format='xml-platform' mapping='logical' distinct='false'>
<entity name='account'>
<attribute name='name' />
<attribute name='telephone1' />
<attribute name='address1_city' />
<attribute name='primarycontactid' />
<attribute name='statecode' />
<attribute name='accountid' />
<link-entity name='contact' from='contactid' to='primarycontactid' visible='false' link-type='outer' alias='c'>
<attribute name='emailaddress1' />
</link-entity>
</entity>
</fetch>
{% endfetchxml %}
<div class='view-grid'>
<table aria-relevant='additions' role='grid' class='table table-striped table-fluid'>
<thead>
<tr>
<th style='width:32%;' aria-label='Account Name' tabindex='0'>Account Name<span class='sr-only sort-hint'></span></th>
<th style='width:10%;' aria-label='Main Phone' tabindex='0'>Main Phone<span class='sr-only sort-hint'></span></th>
<th style='width:10%;' aria-label='Address 1: City' tabindex='0'>Address 1: City<span class='sr-only sort-hint'></span></th>
<th style='width:16%;' aria-label='Primary Contact' tabindex='0'>Primary Contact<span class='sr-only sort-hint'></span></th>
<th style='width:16%;' aria-label='Email (Primary Contact)'>Email (Primary Contact)</th>
</tr>
</thead>
<tbody>
{% for account in accountFetch.results.entities %}
<tr data-id='{{ account.id }}'>
<td data-attribute='name' class='details-link has-tooltip launch-modal'>{{ account.name}}</td>
<td data-attribute='telephone1' class='details-link has-tooltip launch-modal'>{{ account.telephone1}}</td>
<td data-attribute='address1_city' class='details-link has-tooltip launch-modal'>{{ account.address1_city}}</td>
<td data-attribute='primarycontactid' class='details-link has-tooltip launch-modal'>{{ account.primarycontactid.Name }}</td>
<td data-attribute='c.emailaddress1' class='details-link has-tooltip launch-modal'>{{ account['c.emailaddress1']}}</td>
</tr>
{% endfor %}
</tbody>
</table>
</div>
My initial intention was to inject directly to the Insert Entity Form using a liquid tag {% include %}. Not sure why but this was not working as I expected, might be due to the type of data or the timing which things gets evaluated. To overcome this issue I created a Web Page + Page Template to show my table (without header/footer):

Now I am adding a JS to the Insert Entity Form, making an Ajax call to that page, and appending to section in my form:
$(document).ready(function () {
AddAccountList();
});
function AddAccountList()
{
try {
var url = "/account-custom-list/";
$.ajax({
method: "GET",
url: url,
success: function (data) {
if (!!data && data.length > 0) {
$("table[data-name='Accounts']").append(data);
}
},
error: function (jqXHR, textStatus, errorThrown) {
console.error("Error XXXX: " + jqXHR + ", " + textStatus + ", " + errorThrown);
}
});
} catch (e) {
console.error("Error XXXX: " + e);
}
};
Result:

I hope this works for you, please let me know.