I have also been looking for the answer to this question. The only thing I could find was that, in case I'm looking for something in order line template, I query the sale_order_line table for all the available columns. There is a company_id column in the sale_order_line table. So we can access company name as "l.company_id.name", as illustrated in the following code:

<tr t-foreach="o.order_line" t-as="l">
                                    <td>
                                        <span t-field="l.company_id.name"></span>
                                    </td>

</tr>

But I guess you have already tried this.

I'm also searching for one that would give all available attributes - it would speed up report development a lot.