It is not possible for a LN "hub" to run on fractional reserve under the LN protocol as it exists now.

When a user opens a LN channel with a "hub" (or otherwise), the user will use an input worth "y", and the "hub" will use an input worth "x" to create a transaction that opens the channel (x and y may be equal, but they do not have to be), and the initial tentative closing transaction will be so that the user receives y and the hub receives x. It is not possible for the "hub" to get the user to reduce the value of their tentative closing transaction by a certain amount without the user agreeing to as much (and when acting economically rationally, without receiving a similar valued amount via other means).