Since TCP protocol requires a connection to be established between two locations before communications can commence, I would think that this is not possible. Maybe there is a Modbus/UDP mode that would allow broadcast.
Sorry - this a few months old.
Since the Modbus/TCP spec demands use of TCP/IP, one could not formally use IP-based "broadcast" to 255.255.255.255 or even a subnet broadcast as that would require use of UDP/IP. Some products allow using UDP/Ip to move Modbus/TCP formating.
However, the growing importance of Ethernet and IP-based networks in factories means using IP broadcast for "normal protocols" is frowned up and any professional in charge of a factory-floor network would NOT use such a product. Broadcasts load every device on the network, and can cause product "A" to affect/kill unrelated product "B". In other words, turning on a CNC machine could bring down a security camera, etc.
best regards - LynnL, www.digi.com