Looking into this a little bit further it can be done but I’m not sure it will give you the results you are looking for.
When you create your translucent material set the TLM mode to Surface and the blend mode to Translucent. The problem here is that you will not really be able to see it without the mesh that is casting the shadow being scaled up. It’s a very soft shadow and when it’s a fully translucent object you will not see it at all.
I know you said you are using a 2D isometric background, is this an image you’re generating by making the 3D models and then making the 2D image or are you getting the 2D image from another source?
It was suggested that if you’re making the 3D models you could use them as Imposter Sprites to generate shadows that way based on the models themselves. I’ve not tested this method but it may be a possibility.