The ability to add custom buttons to forms programatically using the forms ribbons should be supported in MS GP addins, as should be access to the GP menus.
Although there is a 3rd party solution to allow access to the menus, it is a big omission from base functionality in my opinion and I have not seen solutions for access to the ribbons.

Using modifier to place extra action buttons to forms is not really an option since the version of GP where the ribbon was introduced. It does not work well with the different ribbon modes the user can select, nor is it good for web client.
Native access to create and handle extra ribbon buttons should be added to the SDK for Visual Studio Addins.