Need Action on Record Type Page

@saratt To the best of my knowledge, what you have been looking for is not possible as there isn't an option yet to customise the Record List Page Type's content. There might be few workarounds such as, having the Record List as a Grid in a Report and a Process Launcher in the same Report under the Grid, but these comprises the effective OOTB features(such as searching and filtering) which isn't appreciable.

And to add, as long as 16.2 or earlier version is in usage, IMO, usage of Start Process Smart Service or a!startProcess() function is recommended when compared to Start Process Writer plugin. I believe you are aware of the fact that these features lack the ability to surface a user interface. Both the Shared Component and OOTB functionalities are good at triggering process asynchronously and also, they won't make the user navigate as the process is triggered asynchronously. In case if you are expecting a user interface to be seen after triggering the process, you could try using the URLs(which are environment dependent) but you need to check whether this way of triggering the Action is moving the user away from the Site or keeping the user within the context of Site and bringing the user back to the Site post completion of process.

@saratt To the best of my knowledge, what you have been looking for is not possible as there isn't an option yet to customise the Record List Page Type's content. There might be few workarounds such as, having the Record List as a Grid in a Report and a Process Launcher in the same Report under the Grid, but these comprises the effective OOTB features(such as searching and filtering) which isn't appreciable.

And to add, as long as 16.2 or earlier version is in usage, IMO, usage of Start Process Smart Service or a!startProcess() function is recommended when compared to Start Process Writer plugin. I believe you are aware of the fact that these features lack the ability to surface a user interface. Both the Shared Component and OOTB functionalities are good at triggering process asynchronously and also, they won't make the user navigate as the process is triggered asynchronously. In case if you are expecting a user interface to be seen after triggering the process, you could try using the URLs(which are environment dependent) but you need to check whether this way of triggering the Action is moving the user away from the Site or keeping the user within the context of Site and bringing the user back to the Site post completion of process.