Hello and welcome,
The loans feature has been requested a bunch of times, and it is indeed a ubiquitous real-life mechanism that should find its way into this site .
I thought of solving the problem after I implement the tags in inventory feature, as described here: http://deckbox.org/forum/viewtopic.php?id=67. So you could create a Loaned Out tag that you would apply to cards you loaned. And maybe you can also create tags for each person you loaned cards to, and tag a Lightning Bolt with wahonez and Loaned to mean that you loaned it to that person.
There's a big problem with this approach though. You will not be able to say you have loaned exactly 2 Lightning Bolts to a person, as tags will be applied to all cards of one type you have.
Another very simple approach I thought of would be a Notepad - a page you can edit and write whatever you want, that only you can see. (with rich text - bold, images, links, lists). That would allow you to have your own system of taking notes, but it has the downside that there's no Inventory integration... Maybe the notepad thing will be a temporary solution.
As for the proper, intelligent solution to this problem... I haven't found it yet. But it's high on the list of priorities
Feedback - as always - is much appreciated.