In later Forge 1.7.2 builds and 1.7.10 on-wards, Forge does a check between client and server for missing block mappings when a player joins. Only after that check does Forge fire events for player/client join. Obsidian sends addons that add blocks/items when a player joins.
This is a classic chicken and egg issue that is a blocker. We need to figure out a way to download/initialize addons before the mappings check.