The Z offset for your avatar can be adjusted via the small spinner in the tiny "Movement control" floater (open it from the View menu).
You can also add command-lines to the chat via Lua. An example is even given in the
Lua manual (Appendix A, page 49 for that command, currently) on how to add a "/z <offset>" command.
Please, before posting feature requests for a viewer you are just discovering, try and make a proper acquaintance with it and its features...