The developer will not be able to select a broker to publish. The default broker will be configured in IS and all the published documents will go there. If you want to route a published document to another broker then you can do that using broker territory and gateways.
When using the pub.publish services IS uses the Broker configuration established using Administrator. This configuration specifies which Broker to use. IS supports connecting to just 1 Broker.
Using the JMS adapter, and the JMS facilities of Broker, I believe you can configure services to use multiple Brokers (e.g. serviceX connects to BrokerA, serviceY connects to BrokerB) but I’ve never tried that.