I had a biorb once.. The baby version to be exact. IMO, the filter thats integrated in the base was poor and I guess only ran by the air pump. Just doesn't compare to a pump (impeller) driven filter.
Anyway, the ceramic rocks act like a biological filter (similarly to the bio balls in a 3-stage filter) due to their high porous structure. If you remove them and replace with sand, you're not gonna have a suitable place for colonies of beneficial bacteria to thrive and do their nitrifying duty. Secondly, by adding sand, the integrated under-gravel filter wouldn't work because water wouldn't be able to flow through the sponge and activated carbon contained within. You'll definitely have to add a separate filter if you do this. This DIY project of yours sounds interesting. I just don't know how it can be achieved while maintaining the aesthetic simplicity of how the biorb looks. I mean, if you add a filter, wires are going to be hanging out the top? Not to mention the challenge of somehow hiding the new filter or making it appear less noticeable. inquiring minds would like to know...Good luck!
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