Lets look closer to see how this works behind the scenes. I am not sure there is an available platform that makes this feasible. I will describe how to implement it in Keykos (not currently available) and invite advice on whether current browser environments support such function. I have the plug-in documentation for building plug-ins for Netscape on the Mac. I may have some comments on why I find this inadequate to learn if this is feasible.
In the Keykos (or EROS) environment I would expect a browser to invoke a factory in place of a plug-in. The factory would create an instance, called the Slot, associated by the browser with the visible result of an EMBED html tag. An initial invocation of the new instance would provide a capability to draw in that screen area along with tag keyword values. I expect that the Slot would draw a picture of a slot over an amount.
The user’s notices the coin-slot and invokes a non browser application which is a coin purse object. This is conceptually a long lived object that owns some screen space sometimes. The user enters an amount in the purse’s window and a coin of the value is visibly produced. The user drags this coin to the slot and drops it.