I do not want to post which house or how to do it here... if an admin will PM me I will explain what it is. I was able to access chests and target people inside while riding a polar bear on a certain player owned structure. This may be common knowledge and already known, but just in case I won't go into detail here on forum... since I have one and macro my characters in it.
Its unlikely that this behavior is abnormal compared to other mounts but if you want to contact me on IRC or via a conversation here I can run some tests. You should also simply switch to a horse and repeat the tests. I suspect the results will be the same. Depending on the house some containers can be accessible, and some locations can be targeted. This is why we have the protections on houses for area of effect spells, etc.