I would imagine that inverting the custards onto a platter and then
trying to move them to a serving plate is frought with hazards.
Custards do not like being moved from my experience.
You may end up with a whole batch of 'cracked' custards for your
trouble.
I'd be inclined to hunt around packaging stores for a disposable creme
brulee type mould. (similar packaging as noodle box - watertight)
Or scounge at a local caferteria or school to borrow 50 moulds.
Or, given you'll be using a bain marie to cook them, try a test run
with some short glasses which are relatively cheap and will have the
sloped sides you need during inverting. No great need for pyrex when
cooking in a water bath at the temperature of the oven is generally
very moderate to low.
You could always sidestep the inverting onto serving plate, serve in
the container caramelising the top like a creme brulee... then you
wouldn't have to worry about perfect sides, inverting etc - but you'd
have to have 50 separate containers..
How large are the desserts? Maybe those disposable shot glasses would
be ideal?
I'd definitely stick with the water bath method.

LadyJane
--
"Never trust a skinny cook!"