After digging a bit more, I realized that this is mediated by the canBeAbandoned​​ getter within @web/model/record.Record​​, which is itself mediated by the Record.dirty​​ attribute.

I was able to achieve my desired behaviour by implementing a custom widget extending X2ManyField​​. I overrode onAdd​​ to set dirty​​ on the newly added record if an option in the context was set.

Here's the code: 

/** @odoo-module **/

import { _t } from "@web/core/l10n/translation";
import { registry } from "@web/core/registry";
import { makeContext } from "@web/core/context";
import {
    X2ManyField,
    x2ManyField,
} from "@web/views/fields/x2many/x2many_field";

export class LineItemX2ManyField extends X2ManyField {
    setup() {
        super.setup();
    }

    async onAdd({ context, editable } = {}) {
        context = makeContext([this.props.context, context]);

        // call the normal onAdd function, but if "forceDirty" is set, force the dirty
        // field to true so the record isn't immediately deleted upon no interaction.
        //'
        // we use this for records that are added with sensible defaults that may need
        // no tweaking, and can be left alone.
        return super.onAdd(...arguments).then((_) => {
            if (context.force_dirty) {
                this.list.records[this.list.records.length - 1].dirty = true;
            }
        });
    }
}

export const lineItemX2ManyField = {
    ...x2ManyField,
    component: LineItemX2ManyField,
};

registry.category("fields").add("line_item_x2_many", lineItemX2ManyField);