If you use a disposable piping bag you can pop it into the microwave for 10 sec increments to soften it, just need the whole thing really really well after each 10 sec. Hope it helps!
I must have missed your post... I leave my ganache on the counter over night and use it at room temp. Only when I am storing it do I put it in the fridge. I don't know what ganache buttercream is though.... are you adding ganache to buttercream?