we have implemented the "POS" module to one of our customer, similar to yours but we do all things within the admin pages, let me know if we can cooperate.
Think that is a great way of doing it, and we did give it a lot of thought. The reason we didn't though is we have a lot of special requirements that were hard to fit into it, plus we didn't want the user to have admin access. Many of the items we sell have a website price and have a different price in store. We also have hazardous material that the buy button is disabled with so web site user can't buy it, but we want to sell only in the store. The other thing is we do a lot of 'one off' special orders for items we want to track, but not list on the website, this happens several times a day and is hard to work around.
Since reporting is a big deal too, we will give this a lot of thought and see if we want to move forward with this project or not.