Currently it is not possible, this is a limitation of the Octane engine core architecture, but once the OSL support is in place, the Octane team wants to add this feature to Octane ASAP.
We need the OSL support first because currently the available shaders are not going to be able to access the attributes data. Using OSL we are going to be able to write any kind of shader node, including the nodes needed to read the custom attributes.
The team is aware about this important feature, so I hope to see it along the 3.x dev. cycle.
-Juanjo