Former barista, it _is_ an unusual order from the perspective behind the bar. Unusual in that I never have heard of someone ordering a warmed cold brew.
I'm not certain how I'd warm it up. I suppose it could be poured into a clean frothing pitcher and steamed directly, I'd somewhat worry that might dilute the flavor of the coffee.
I don't reckon someone's going to want me to microwave their cold brew, but it certainly seems like it'd be the quickest way to do it.
Most cafe workers get into flows of orders. Lattes means you always pump syrups into the cup, start the pour, then steam milk. Cold coffees usually means you ready the cup (syrups, milk) and pour the cold brewed coffee onto it.
Warming the cold brew totally breaks that flow, and is why it would be unexpected.
Hope that perspective helps. I do want to try it now though! I could imagine it being pretty good.
