Is it easy to get old versions of Bitcoin?
You wouldn't want to. There have been changes that would tend to give your transactions lower priority. In particular, large transactions will take a long time to be accepted because the allowed size for free transactions has been decreased significantly and now network nodes won't relay transactions that they feel have too-low fees.
It's an easy change to make in the source, though:
if (!pcoin->IsFinal() || pcoin->fSpent)
return (SelectCoinsMinConf(nTargetValue, 1, 6, setCoinsRet) ||
SelectCoinsMinConf(nTargetValue, 1, 1, setCoinsRet) ||
SelectCoinsMinConf(nTargetValue, 0, 1, setCoinsRet))||
SelectCoinsMinConf(nTargetValue, 0, 0, setCoinsRet));
(Add a line that allows 0-confirmation coins)
Transactions using coins with few confirmations have a low priority for generators. Make sure you keep lots of wallet backups if you do this -- you might end up having to double-spend your own invalid transaction.
The best solution would be to change the sending priority. The client would first pick confirmed coins, since unconfirmed ones may remain like that for an indeterminate time. Maybe a warning message when there are only unconfirmed coins to be sent could be useful too. But completely blocking sounds "too authoritarian" to me... it's not up to the software to decide that.
You can change the behavior, as above. That change will prioritize as you described.
If I mine a block with my custom miner and accept to put <0,01 transactions in it, the default client won't refuse them, right?
Right. That's the only way to do it right now -- I've not heard of any major miners accepting sub-cent transactions.
Also, one doubt that I once had... can somebody else than the send of a transaction add fees to it?