The curve sample node should have a field input that is evaluated in the context of the curve geometry, and sampled like the other inputs.
The values should then be saved as an array in the multi-function, and interpolated to the evaluated points of its splines.
Then the multi-function can sample like the other outputs, only with a bit of trickery with the signature to support all attribute types.
The node will need a type drop-down for the type of the new input and output.
The name of the output and input sockets can be "Attribute", and the value should be hidden for all new input sockets.