Ok, this is something I would solve by manipulating the GridEdges map variable in the Grid Manager. GridEdges holds arrays of which tiles can be reached from each tile on the grid. All you would need to do is to remove all edges that point back towards the start. There might be a simpler way to do this, but here is the first thing that came to mind.
Add an invisible wall between the start tile and the end tile. Then call RunPathfinding starting on the start tile and with a Move argument equal to or greater than the full distance in tiles between the start and the end tile. Keep the Paths output for the next step.
Next loop over GridEdges and do a nested loop over each of the nested arrays within the values of GridEdges. For each edge check Paths and find the cost. If the cost of the tile the edge connects to is lower than the cost of the source tile (meaning that it is closer to start), remove the edge (use the RemoveEdgeBetweenTiles function). Lastly when all is done add a new edge between the end tile and the start tile and you have your Monopoly board.
That was my first thought. See if you can figure it out, and if not I can give it a try tonight.