Can I seal my bath overflow waste with silicon, from the inside?
It will not work. 

You will need to find the exact cause of the leak (e.g. disturbed slip-joint fitting, cracked tube, fitting pull-out, bad gasket, etc.) and fix it proper. If it is "freestanding" then the overflow should be accessible unless it is concealed in a skirt. If it's concealed... time to disconnect the supply and drain and tip that b^*ch up...