Initial data broadcast upon subscribing to a channel (onSubscribe)

Hi, I'm using Reverb to let external sources listen to events from my website. Everything works fine, but they want to have an initial set of data to present before the first update has been broadcasted.

I can't find any information of an onSubscribe function or anything similar. Is there a way to do this? Or do I need to build a separate data feed they can request initially when loading their page?

Thanks