How do you change atomic valence in Avogadro? I have a bridging hydroxyl group between two metals, so the oxygen is triply coordinated in this structure.

How do I change the valency to three in Avogadro so that these bonds to the metal are displayed?

Perhaps it might be necessary to also play around with the van der Waals radius.
Or is there a direct way to enforce a drawn bond between two atoms?
While there are some automatic valence features in Avogadro, you can always turn them off. Bonding is mostly a matter of appearance.

For example, let's import $\ce{IF7}$.



Notice that the central iodine doesn't show enough bonds. So we'll draw some in. Switch to the draw tool and turn "Adjust Hydrogens" off.



Now just draw connections between the atoms. Voila!