This feature is available in version 1.3+

Go to admin > rentals > manually reserve inventory

Here you will be presented with a list of all reservations whether they are manually reserved or not. Manually reserved inventory will show the order number as 0.

Edit Rental Dates #

click on the reservation you want to edit. This can also be done from the order view > Edit Rental Dates button.

From this page you can change the rental dates.

Manually Reserve Inventory #

From this same page click the Add New button.  Here you will be presented with a form where you can enter the rental dates, quantity, and select the product to reserve. Then click save.