Not that different from your work-around, but what I do for things like that, is I have actually set up a "TEMP_LINE" FC in my SDE and we use for tasks like that so we don't have to create a copy of anything and such as that. I just add a feature to the TEMP_LINE FC using the trace tool. Because it's in an SDE it auto calculates the feature length. Additionally, if you need to change units, you can always do that in the field properties using the already provided shape_length field. You just open up the layer properties, go to the fields tab, select the field, and click on the number format (on the right side) and edit this using the "Rate" option. Using that method, you can have the FC in a projection that uses, meters let's say, and have it auto display the length in feet for you so you don't even have to manually convert it.
Ok, so, I know that's still more work that it seems like it should be because the measure tool should be able to use trace feature, I completely agree. But, that's the easiest work-around I've found. At the end of the edit session, I can even just open up that table and delete the temp line features, or, I have added a "project" and "label" field to that, and I can also save those measurement lines that way and use them for display in presentations, maps, etc... as needed (I just use a definition query on the project field to get the ones I want).
Hope it helps and makes sense, if not, let me know and I'll try and clarify what I mean.