I've seen it done. Something like this. Using RSP accounts, you and some friends (not relatives) buy into a small business so that all are at arms length. It has main activity in Canada, but it also owns a sub in the US. The sub is simply a trading acct with a US broker. The Sub makes money which is transferred to the parent. The parent makes money from the sub. It is a long way around to trade US stocks in a RRSP, and is now made obsolete with new foreign content rules. plus its tough to find an eligible business.
However, a similar strategy is very logical to avoid taxes outside a RSP, but not so swell for a RSP for other complicated reasons I'm too lazy to explain.
Forget the RRSP, from a tax saving point of view, this same set-up is used, and you as an idividual acquire a Canaidan co with a loss, and if you know how to trade, then you can create a profitable US trading sub of the Canadian loser, and shelter Cdn losses against US profit. Done everyday. Net result, tax free earnings.
By way of explanation, a RSP is basically a Canadian version of an IRA
Hope that made sense.
Peter