What you want to do is neither doable with LOTRO options or a plugin.
But it is doable with the Steelseries App for your mouse. Unless you consider software from the manufacturer of your mouse third party. In that case, there is no way for you to do this.
What you need to do is use the Steelseries app to create a profile for LOTRO, in that profile map, for example, the mouse button 7 to "4" on the keyboard and then pressing that mouse button will be understood by the game the same as pressing the "4" on a regular keyboard. You can map your mouse buttons to almost everything found on a keyboard.
This is a) how all gaming mice do it and b) what is described in the manual of your mouse:
http://www.subdude-site.com/WebPages...2010_15pgs.pdf