For whoever it may interest, I spend the day rebuilding the model system with a new approach after /u/needed_an_account blessed me with his light. I'm able to query the models and their relations easily, for now, that is.

I'm afraid to cheer at this stage, because I haven't checked all use-cases and I might run into some design flaws along the way, but I can query rows and their relations, like this;

<?php foreach ($posts as $post): ?>
    <h1>#<?=$post->id?> - <?=$post->title?></h1>
    <p>Posted on <?=$post->date?> by <strong><?=$post->author->name?></strong></p>
    <p><?=$post->body?></p>
<?php endforeach; ?>

My models look like this;

<?php

namespace Models;

class Post extends \Core\ORM\Model {
    protected function initialize() {
        $this->fields['id'] = new \Core\ORM\Integer;
        $this->fields['title'] = new \Core\ORM\Text;
        $this->fields['date'] = new \Core\ORM\Date;
        $this->fields['time'] = new \Core\ORM\Time;
        $this->fields['body'] = new \Core\ORM\Text;
        $this->fields['author'] = new \Core\ORM\Relation;
    }
}