Ya know, it's better to create your own threads after a while like that.
But anyway. I don't know much about the specifics, but many men get manucures, and it doesn't look off. So I think the easiest thing to do would be to go in in boy mode and ask for a manucure, simply. They'll do you the "man-ucure" (bad joke ^^'), which is just clean-looking nails, pretty short, with clear polish.
If not, you can just go in and ask for something discreet. In general, if your nails aren't all that shiny (matte-ish polish), not too long and not coloured, it shouldn't attract too much unwanted attraction. Then again, it might not be what you want, but the way I understand it, you still want to pass as a guy without too many questions, yes?