I myself can't say for certain, but I do know SideQuest/EAS never says never, and they always leave chances open for any possibility. While it's far too early to say whether a Rainbow Moon 2 could be developed, it's definitely a possibility.
I remember constantly checking for Soldner-X 2, and one night my search results turned up with something - and I was ecstatic. Then they made DLC, which made me even happier. Because it was in the hands of SideQuest/EAS, it's turned out to be one of the best shmups ever made because of its sheer quality in every aspect.
So, I imagine if the same thing happens with Rainbow Moon, you can expect a sequel. I will say that you can certainly expect DLC (although there're no specifics on this yet).
It's also important to note that Rainbow Moon is set in the same universe as Soldner-X, simply in a different era (Ellje Woods is a dead giveaway). This means that SideQuest/EAS may simply expand on the same universe through different-genre games (and sequels to them). To me, building a complex, if even loose, continuity would allow them to have their own 'distinction', something to associate themselves with (for example, with Nintendo you have Mario and all the settings he's been in, etc).
So in effect, I don't think it's been discussed yet, as mostly they're focusing on making sure that what they do have right now is up to PSN standards in the first place. After everything is said and done, you can best bet SideQuest/EAS will deliver.
I myself am looking forward to what they might have up their sleeves with the PS Vita (COUGHCOUGHyou have no idea how many times I've wanted to play the Soldner-X games on a handheldCOUGH), but as I've said before, no matter what they decide to do, I'll always be there.
So just keep your fingers crossed!