Hi,
You mean the Timeline sample right?
Note if you click on Github link you'll notice it will direct you to a deprecated CORE version. So this visual as some others were previously built using the CORE version which is basically an application built in Visual Studio 2015. I haven't seen any updated CLI code source version.
So in order to bring modifications to any existing pbiviz you have 3 options:
1 - as you mentioned, you may try to decompile the pbviz (changing pbviz to zip - bring modification
like changing GUID then recompile it). However this approach may NOT always work as expected.
The approach has been mentioned a few times... though to be honnest I wouldn't bet a million do$ on it.
2- next approach would be to take the available code on Github and bring modifications right at the source.
The CORE application doesn't provide any pbiviz command line righ out of the box like PBI CLI pbiviz package.
Though I managed to create pbiviz file through Gulp package method. Following link gives an example how it can be done.
http://amolpandey.com/2016/08/02/powerbi-how-to-create-pbiviz-file-custom-visualization/
3- last option well is you feel comfortable enough is to rebuild a new CLI version from scratch then be my guest.
Hope this helps