There isn’t an easy way to set the layer in the legacy connector.
You’d have to place objects within a Collection class programmatically, where the name of the collection corresponds to the name of the layer. For blocks this is even more complicated: you would need to include a dynamic layer property on the geometry inside your block definition to get it on the correct layer.
You could try this with the extend speckle object node in the legacy connector, before you pass your geometry to the block definition, and see if it works - this may get your definition objects on the right layer, but not the instances.
We currently don’t have block support in the Next-Gen alpha release of GH, but will be adding it for the beta. After beta is released, you should be able to easily add block geometry to specific layers with the Create Collection node.
See this thread for reference on both: https://speckle.community/t/creating-collections-in-c-speckle-sharp-rhino-gh/18049/6