I basically want to use a short as a hedge while investing in altcoin-denominated investments/securities.
I am not finding any exchanges offering selling-first types of trading. I have seen this feature only with Forex trading platforms. I believe, for cryptocurrency world it is completely new thing. Basically cryptocurrency exchanges are still working peer-to-peer type of exchanges whereas forex works peer-to-broker kind of exchange.
My long term investment goal is to grow my BTC position (regardless of current exchange rate in terms of USD).
For making more bitcoins, I guess you need not looking for shorting altcoins. Making use of altcoin trading and just holding bitcoins will help you to earn better in more effective way within short period of time than you expect.