Changing "Login into store" behavior - feedback requested

Greetings -

One of the use cases for my installation of OpenCart is to be used as a Point-of-Sale system. I purchased a POS extension, and its out-of-the-box features are really good, but I really need to combine the "Login into store" behavior with it so that the POS user can create quotes, modify orders, etc., on behalf of the customer.

To that end - I've noticed that it's really easy to Login into store multiple times as different customers, and casually leave those tabs open. They can easily start conflicting with each other ("customer #1's tab changes customer #2's info, etc."). In short, I'd like to change the workflow of Login into store to open one customer session in an iframe over the customer admin area (covering it up, in essence), and not (easily) allow a second Login until the first is closed.

In my casual searching about OpenCart and iframes, I'm seeing "don't do it!" warnings, mostly around Paypal stuff. However, I don't have a need for Paypal or the iframes it uses. Thoughts? Is there a better non-iframe way of accomplishing what I want?