I think George's answer explains fairly well why the brokerages don't allow this - it's not an exchange rule, it's just that the brokerage has to have the shares to lend, and normally those shares come from people's margin, which is impossible on a non-marginable stock.
To address the question of what the alternatives are, on popular stocks like SIRI, a deep In-The-Money put is a fairly accurate emulation of an actual short interest. If you look at the options on SIRI you will see that a $3 (or higher) put has a delta of -$1, which is the same delta as an actual short share.
You also don't have to worry about problems like margin calls when buying options. The only thing you have to worry about is the expiration date, which isn't generally a major issue if you're buying in-the-money options... unless you're very wrong about the direction of the stock, in which case you could lose everything, but that's always a risk with penny stocks no matter how you trade them.
At least with a put option, the maximum amount you can lose is whatever you spent on the contract. With a short sale, a bull rush on the stock could potentially wipe out your entire margin. That's why, when betting on downward motion in a microcap or penny stock, I actually prefer to use options. Just be aware that option contracts can generally only move in increments of $0.05, and that your brokerage will probably impose a bid-ask spread of up to $0.10, so the share price has to move down at least 10 cents (or 10% on a roughly $1 stock like SIRI) for you to just break even; definitely don't attempt to use this as a day-trading tool and go for longer expirations if you can.