Bluz only operates as the peripheral, or client in your example. The central is the device that can scan for and connect to peripherals. The bluz gateway and gateway shield are central devices, they are also open source and so you could technically use them for that purpose. However, that would require you to local compile and change the firmware to look for new services and characteristics.
You could theoretically use bluz for this purpose as well, but it would require a JTAG programmer to completely change the code loaded on the device. We use Nordic chipsets and their BLE stack comes in two flavors*, one that supports central and one that supports peripheral. So you would have to change the underlying stack, which requires a JTAG programmer and some definite firmware changes.
*Actually, Nordic does offer a third flavor of their BLE stack that can act as central and peripheral at the same time. We don’t use this for bluz since it has a lot more limitations, so I’m not including it in the conversation here since we don’t use it in any way.