If need find timecode clipitem it's easy. Take TC source file and plus in and out tags.
But how made it if we have Time Remap?
Where can I find formulas for calculating TС In / Out?
I have no clue if this applies in general, but in this case, the last keyframe of the speed graph is equal to the source clip's duration. Maybe the <when> values refer to the source clip frame numbers, that'd permit you to calculate the relative source In/Out duration. Hope I've understood correctly what you're after...
I tried <when>, it seems they are specified for In/Out. On this example I did it.