I need to install some new fence panels into concrete posts, the kind with a slot intended to allow you to drop panels in from above, however there are overhanging trees that will render this traditional method of installing the panels impossible. I've tried removing the strips of wood at the top and bottom of the panel that keep it rigid and bending them in, but I'm unable to bend them far enough to get them into place (maybe with assistance I'd be able to do this, but unfortunately I'll have to do this job by myself). Are there any other tricks I might be able to use to get the panels into the slots?
Cut the panels (horizontally) into strips short enough (vertically) to slip between the limbs and the top of the posts. Stack as many strips as needed to get the full height.
So if you have 2 feet of clearance and 6 foot tall panels, cut them in to 3 strips, insert each strip. If you have 3 feet of clearance, cut them in half. If you have 18" of clearance, 4 parts...one variant of the type of fence you describe uses strips (boards or sometimes concrete "boards" when going for a "never have to do that again" method) that are only 6" or so tall. Stack 12 of those and you've got your 6 feet (tall.)
If the tree branches wont bend and cannot be cut, then I would suggest a more unorthodox approach which is to make the fence panel flexible. Can you cut it and apply some hinges so it remains together but can be bent enough to get into the posts?
You mentioned the top and bottom beams of the panel were removed, so I would assume when putting those back the panel will then be rigid again.