i dont know whats so hard about loading a game and then plopping in a quarter.
the only thing i can see this useful would be for like someone said nintendo vs, or neo geo.. but those are things i think MAME should implement them selves, it's so lonely looking at a nintendo vs screen with 1 game on it.
anyway i suspect it's done in one of two ways.
1, put quarter in, when game loads it waits for x amount of seconds then triggers coin drop for amount of quarters inserted.
2. put quarter in, then a button not used for anything else is triggered and held down.
mame is setup for the coin drop, to be triggered when ever that button + start is pressed, this could definitely be done using hardware but could probably just as easily be done in software.
the program keeps track of how many times start is pressed, when the presses have equaled the amount of quarters that have been entered it releases the button.. so start no longer is able to trigger more coin drops.
i've done setups where coin and start are assigned the same button it does work although on some games it has to be hit twice.. some games dont like having coin + start at the same time.. most likely start is being registered first before a credit is entered.
you could probably combat that with having the program take over all start button entrees.. so in cases of credits it could send coin drop just a few ms before start.
if you want a led display you could get one out of a old digital clock, or off a old AT pc case, you know the kind that had their mhz displayed on the front.
then program the LPT (printer port) for output to the led's.
thats how i'd do it anyway, but again why would anyone want to do this?
even someone who was gonna put a mamecab on a route?
infact "IF I" was going to do something like that i would wanna hide any trace of mame, and just have a single game load, with no way of exiting the game.. from the outside i'd want it to look exactly like the real deal.
just imagine 2 people are playing and one gets mad and hits the exit button.. someones getting their ass kicked for real, good way to start fist fights.
if you wanted to mix it up a bit you could always set it to randomly load a game on start.. then every day (assuming they shut it off at night) would be a different game.. or have it set to switch games when the place is closed.. like 2am.
although i honestly dont think it's worthwhile to use mame on a route.
i've been told im wrong on this.. and i've never had a arcade machine on a route making money so i suppose there might still be money left to be made.. although i can't imagine putting time and effort into a mame cab only to set it up in public for abuse.
The commercial aspect would be the biggest concern to me.
Seems like when anyone NEEDS something like this it's because they want to use a system commercially somehow.
My $0.02
+1.. also anyone who has a working coin door are surely gonna set it up for pay for play.. i totally agree.