@Robert H
So I looked into this last night. It doesn't appear currently possible because of either a design limitation or a bug in DAS.
The way I was going to do it was to store the double-clicked value (which gets filled into the montage PRICE field, the "initial stop") in the "StopPrice" variable (switching to ROUTE=Stop and back again). However, it appears that when ROUTE=Stop, the "price" variable is set to "StopPrice", so it's a circular reference (doing StopPrice = price is doing StopPrice = StopPrice). I couldn't find any other places to store the initial price temporarily, as the only floats are available in the STOP route. I also tried converting the datatype, but DAS doesn't support it.
Contrary, when ROUTE=Limit, StopPrice is still a valid variable to use. So I'm not sure if this intended or a bug - doesn't seem like it should be this way, as it only has limitations and no upsides I can see.