PoS, which is what a masternode is, is generally considered "mining", as far as I know.
That said, I think where Dash could have a reg. issue is their whole governance system. They have very strong leadership from their foundation which is basically voted into existence through their budgeting system. I think the SEC would have to conclude (unlike the DAO) that the budgeting system at least is truly organized as a decentralized organization. That said, that decentralized organization has basically handed off the reigns to a centralized organization operating in the USA with office space. And they run their management in that capacity, as a central organization.
That said, I think the mining + PoS may effectively save Dash from being a security. There is a strong inclination in US government that bitcoin is not a security because it is mined. PoS has seemed to fall under this as well. Since voting rights are all derived from a distrubuted mining system where anyone can participate and not an ICO like the DAO, I think the SEC is less likely to try to upset that precedent.
If the SEC did go after PoS algorithms, it would affect Ethereum as well when that switches, and many other PoS coins.
The problem with proof of stake systems is that they could be more easily seen as securities. One of the tests for a security is that income is derived from the work of others. Supposedly Bitcoin and other mined systems don't qualify, but master nodes derive passive gain to their holders and people can create pools of stake and thuis deriving money from others. The same is gonna be true about ethereum and the rest of the projects by the ethereum foundation. At the moment all we know is about the DAO's particular case but who knows what might qualify as a security.
I also don't like much the X11 cryptographic algorithm in Dash and the whole instamined 2 million coins by the creator and all the rebranding of its past (Xcoin, Darkcoin). Only instasend technology in it seems particularly good.
The SEC already called Ethereum a currency, though, in their latest report. Meaning, they'd have to reverse on that statement. That said, it is mined, so if it switches, maybe the SEC eventually switches their stance.
If they do, expect a large ETH crash. Coinbase, poloniex, and bittrex would all have to stop trading ETH with US citizens.