Your right actually, you can't see a monetary liability in a regular expense transaction.
Here is another option for you:
Do a new Discharging Liabilities transaction, choose the Category that you created for Interest as I said before, and choose your Monetary Liability as the account. Don't put anything for interest, but instead put a Negative value on principle using the +/- button on the right after you type the amount. Save your transaction and your Liability will increase.
You could probably also do it as Extra Payment instead. Also since you use the different category just for this interest you can still track it the way I suggested earlier and see how it goes down over the period of your payments. I have only tested to make sure that the liability does indeed go up though.
Hope that helps. I can post screenshots elsewhere if you need.