We do not support extending our software via javascript, only via the SDK. That said, folks do many interesting things (javascript and/or css) with the knowledge that a future version might break it. Developer to developer: The mapping component intentionally tries to abstract away the internals, which includes leaflet. It is unlikely, but we may need to swap that out some day. That's why it's a convoluted to dig out a reference to leaflet.
var leaflet_mapobj = SW.Core.Volatile.WorldMaps[myMapResourceID].leaflet(); // this is the leaflet map object.
Considering that you're putting in a custom html resource on the same view as the map, you can just hard code that to match the ID in question.
If you do something cool with this information, please procure and upload a screenshot.