Exactly. Ronix's newer liners are lasted to fit the last of the outer shell, and I imagine Slingshot uses a similar process. This idea would work, but it's asking for trouble in the form of discomfort.
The liners in my 2010 One's were simple to replace. All you have to do is remove the 2 screws on the bottom of the sole and pull them out. Send an email to Ronix to see if they can get you some replacement liners.