It appears you will need to create a Bonjour proxy to bridge the subnets. See Using Bonjour Across Subnets:
The Bonjour proxy functionality that is built-in to the
dns-sd command line program is the simplest solution to set up initially, but it can be difficult to maintain. Important to note is that although the man page for
dns-sd says it first appeared on Mac OS 10.3 this is not true. It will run on 10.3, but it did not ship with the OS until Mac OS 10.4.
A summary of three approaches is available:
The three ways to provide Bonjour service discovery across subnets or on a wide area network (WAN) are multicast routing, unicast DNS, or a Bonjour proxy for each service on every subnet. Each of these methods has advantages and disadvantages. In most situations, deciding factors will be the upfront effort required to setup the solution and the ongoing maintenance required to support the configuration.