Do bidendates occupy cis position always?
Its not impossible to have a trans-spanning ligand, but they seem to be rarer than cis-ligands due to their increased length and greater potential for steric strain. While en is too small of a ligand to reach trans-positions, it is possible with significantly longer ligands.

It seems octahedral ones are even rarer than their square planar counterparts, but there has been at least one reported synthesis of a trans-spanned octahedral$^1$ that I could find. In the linked paper, they report the synthesis of trans-$\ce{[Ru(Cl)(trpy)(C3SPAN)]}$ where trpy = 2,2:6,2-terpyridine and C3SPAN=$\ce{Ph2PC6H4CH2O(CO)(CH2)3(CO)OC6H4PPh2}$.

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C. Bessel, et al. Chem. Soc., Dalton Trans., 1999, 2281–2292