No replies to this, so I'd guess the answer is "No"
The problem is that the game treats track as a single line, which the trains follow. Thus narrow gauge trains can run on broad gauge track quite happily, but just looking a bit silly, as they follow the centre line of the track. They even run round a smooth curve when the track in modelled as a series of straights - for example the built-in "grey wood clean std 16m", kuid2:60850:38003:1 appears on sharps curves as a series of 16m long straights, but trains follow a smooth path.
For GWR mixed gauge track the broad gauge and the standard gauge (assuming its a 3 rail type) will have different centre-lines. Perhaps the answer is to lay a broad gauge sleepered track, then a rail-only standard gauge track, positioned so that one rail coincides with the broad gauge rail. Does this sound like a solution?