Yes, those are the two ways to create a toolbar button - either JavaScript or Plugin.
You are also correct that you can't do anything with ActiveX or DLL from Acrobat JavaScript - which is the same JavaScript engine as Mozilla/FireFox. owever, many browsers add additional/custom objects (such as ActiveX), which may be what you are thinking of...
Plugins are written in C/C++ and they are documented in the Acrobat SDK. You will need to download that to get building. It includes documentation, sample code, etc. HOWEVER, be aware that in order to develop a plugin for Reader you must obtain a license from Adobe. There is a cost and process for this license and details are in the SDK.
