If I had received the request, I have to say I wouldn't be bothering to check if he wants it smooth or grainy. As you say, he's only seen a photo anyway, and you will be making a rod for your own back. If they are slimlines, which I think you said they are, he's gonna get grainy from me and that's that. That's not just me being difficult, there is a practical consideration too, which is this. ...
Take a piece of cardboard and score it with a sharp knife. Bend the card and the score mark will widen to become a gap. The more you bend it, ie, the tighter the radius, the wider the gap will appear to be. Now consider a tiny piece of oak only 8mm or so in diameter. I know you're not bending it, but none the less, the open-ness of the grain is going to be more apparent than it is on say a substantial fat fountain pen. Therefore, as far as oak is concerned, it is much, much more difficult to get a smooth surface on a thinner pen than it is on a fatter one, and in fact, always of course depending on the particular piece of oak on the lathe, on a slimline you would possibly be struggling to achieve it.
My advise to you, take it or leave it, is to accept that oak is what it is. You will probably find that some will turn out smoother than others in any case, and vice versa, because the character of each blank is bound to differ slightly.
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