I’m not sure if that means it’s not possible to program or that there is an aim for there to be uniformity in the behavior of the features found in the filters (i.e. not creating split clips in the timeline).
I thought about this too and a couple of solutions came to me. The simplest one would just to not have it do anything when the clip is in the source player and only have it do something when the clip is in the timeline. The other option would be to actually have the 2nd clip created and added to the playlist.
I had considered the open MLT file as a clip as a solution to the original question and it is a solution but currently there are some limitations with it. I understand that the open MLT file is sort of Shotcut’s current approximation of nesting. But since this form of nesting is based on mlt project files and not done within the same project that means a second instance of Shotcut would have to be opened, the clip created there and then the mlt project file created. After that you bring it into the main project. So there is a bit of slowness there to the workflow. Also, you can’t do everything you can to a clip to a mlt file opened as a clip such as changing the speed of a clip. Another thing is if I wanted to stop the timer at some point and keep it at the stopped time on screen for some duration then restarting it later on like I explained above, I wouldn’t be able to really do that by opening the MLT file as a clip.
The primary reason I had thought of my suggestion for the Timer filter was just in case there were no plans to develop nesting any further. I imagine that if it will be developed further it won’t be done any time soon and that’s very understandable.