I would like to create a contract that shares mining rewards between multiple accounts.
Can I set this contract address as the target address when running eth-proxy ?
If you are mining on a pool, unless the pool clearly states that you can because they make sure to send enough gas, you can't redirect reward to a contract without risking to loose it if the transaction runs out of gas. The reward would then be returned to the pool and you couldn't get it back.
A contract address is the same as a regular address in terms of the 0x.... goes. I know that for sending transactions, sending to a contract requires a bit more gas than sending to a standard account. The amount of gas depends on the contract itself.
For solo mining, there is no reason sending to a contract would be any different than an account as mining rewards are not deposited via a transaction like they are in bitcoin, they just appear in the account as part of the state transition in that block. (Thanks to Tjaden Hess for this last bit!)
Here is some additional information / sources:
Receiving Mining Rewards
If solo mining, you can use either Account or Contract Wallet as your Etherbase account to receive mining rewards. Apparently the protocol “pays no gas”!
When mining through a pool, it is once again safer to use an Account to receive your reward as the pool operator may not specify enough gas to cover sending to a Contract Wallet.