I saw that in my searching but I just end up with Argument 1 passed to PhabricatorCustomField::getObjectField() must implement interface PhabricatorCustomFieldInterface whenever I try to use it, perhaps because I'm unsure what to actually pass it as the $object other than $this, though I second-guess my second-guessing because ManifestCustomField, which I'm extending, does seem to have the interface the error is claiming I'm missing?

I'm kinda stumped. Here I had embarked on what I assumed was a short journey just to make a custom field formatted as a link, hah!

@keithzg what you might be missing is

$object = $this->getObject()

In the case of a maniphest field the $object is a ManiphestTask instance. For other custom field types it's something else like PhabricatorUser or DifferentialRevision

There is a pretty complicated and not really easy to follow subclass of ManiphestCustomField over here: https://phabricator.wikimedia.org/source/phab-extensions/browse/wmf%252Fstable/src/customfields/ReleaseDetailsCustomField.php

It doesn't do things the normal way but it does show some of the APIs being exercised (or abused, depending on your political leaning.)

For another example, here is a fairly straightforward use of that API which takes place entirely outside of a custom field subclass:

https://phabricator.wikimedia.org/source/phab-extensions/browse/wmf%252Fstable/src/policy/WMFSecurityPolicy.php$147-174

Many thanks, @20after4! That all largely did it; in addition to setting up the object a bit wrong (I knew it was a ManiphestTask instance, but was failing to get the actual object of it), I was also just using essentially my:custom-field rather than std:maniphest:my:custom-field.

That, and setting the field's entry in config/edit/maniphest.custom-field-definitions to view: false, has me 100% of where I wanted to be :)

...well, okay, doing this this way still doesn't seem quiiiiiite right, because now the entry for this field shows up under Details even when there's no value. I probably am missing something in my extension code and need to follow the aggressive exercise closer. Shall fiddle a bit more . . .

Okay, I got it! For future reference if anyone in a similar situation is stumbling across this and wants a fully working example to go off of, my more-than-a-little-cargo-culted code is currently:

<?php

final class GmclManifestCustomFieldFogbugzCase extends ManiphestCustomField {
	private $fieldValue = null;
	
	public function getFieldKey() {
		return 'gmcl:fogbugz-case';
	}

	public function shouldAppearInPropertyView() {
		return true;
	}

	public function renderPropertyViewLabel() {
		return pht('Fogbugz Case');
	}

	public function renderPropertyViewValue(array $handles) {
		$task = $this->getObject();
		$viewer = PhabricatorUser::getOmnipotentUser();
		$field_list = PhabricatorCustomField::getObjectFields($task, PhabricatorCustomField::ROLE_EDIT);
		$field_list
			->setViewer($viewer)
			->readFieldsFromStorage($task);

		$field_value = null;
		foreach ($field_list->getFields() as $field) {
			$field_key = $field->getFieldKey();

			// error_log("Checking ".$field_key);
			if ($field_key == 'std:maniphest:gmcl:fogbugz-case') {
				$field_value = $field->getValueForStorage();
				break;
			}
		}

		if (!isset($field_value)) {
			// https://phabricator.wikimedia.org/source/phab-extensions/browse/wmf%252Fstable/src/customfields/ReleaseDetailsCustomField.php$122 used "" but that still renders, while null does not, I'm probably misunderstanding something but hey this works!
			return null; 
		} else {
			return phutil_tag(
				'a',
				array('href' => 'http://www.gmcl.internal/lethebugz/?'.$field_value,),
				pht($field_value)
			);
		}
	}
}
?>

and the actual field definition for completeness's sake:

{
  "gmcl:fogbugz-case": {
    "name": "Fogbugz Case",
    "type": "int",
    "caption": "Associated/replaced Fogbugz case",
    "fulltext": true,
    "search": true,
    "view": false
  }
}

(Yes, the year is 2024 and we are finally at my work migrating away from the old runs-on-your-own-Linux-server version of Fogbugz, which last officially supports Ubuntu Server 12.04. I set up our Phabricator instance over 8 years ago! People phear change, but Fogbugz finally no longer talks to our database server as of MariaDB >= 10.4 due to some sort of incompatible change to MyISAM, and won't talk to any other storage engine which seems hilariously fragile but about par for the course for server software written in a bespoke dialect of VisualBasic and compiled to Mono...)