Hi Paul,
Unfortunately, there isn't a way to do that in the Custom report currently. You have to pick one (or all) of the accounting methods. It would be good to add a "Tax basis (Assigned)" set of fields, so you can choose this one, and it would calculate based off the assigned accounting method. It would be similar to how you can create Capital Gains reports, and choose either a specific accounting method, or the assigned property. This is something I hope to get into the next major update.
You can however create an "Open Lots" report, and choose the "assigned" property to report on. This will give it to you per lot though, and not for the whole investment.