You already pretty much outlined the caveat on why it's not really possible. If you can't grab the string you need with calculations and scripting, the only other tricks to try are conditional formatting or some sort of object layering (EG placing a white box on top of the first row, and using "hide object when" creatively).
I was playing around with an idea, trying to solve your problem, and I came upon a very weird thing.
It happens both in Filemaker 13 advanced and 14 advanced on my Mac.
Open the enclosed file and just click once in the TextField, (or press the ClickMe button) then watch...
SelfAnimationWTF.fmp12.zip 65.9 K
You might only display everything to the right of, and including, the fourth slash character,
Additionally, you could place the entire string in a tool tip, so the user could see it all if you hover over the field.
Just a thought. The world is full of the them.
Weird, but nice!
It took me some time to note that the baseline offset was > 0
Another thought involving a combination of the following
1 - A monospaced font
2 - A calculated field Right(FilePathField,ValueBasedOnWidthOfField)
3 - ValueBasedOnWidthOfField=getLayoutObjectAttribute(FilePathFieldObjectName,"width")*(# of characters per count of px)
Thanks all for the ideas. I ended up just truncating it enough so, worst case scenario, it would fit. Filled the field with Xs and counted them, taking off a couple just to be safe. Thanks,