If all of my money is currently wagered and the fight in the arena has not occured, can the 'Destitute' scheme card be played on me?
First I thoughy "no", since you call it a scheme. But it is actually a reaction and not a scheme (big difference ) - so yes it can be played on you.
At least I would think so, since you could not pay gold to someone if you wanted too, and so you have no gold (maybe a bribe to another player for not using "Rigging the Match" on you ).
Anyway it must be the same answer for the cards about "no slaves" and "no gladiators", if you had just send in your last slave or gladiator to fight (if they are also reaction cards of course). If these three cards cannot be used at such times (where your assets or gold is tied up elsewhere), they'll have a rather limited use.