I too have the offer of a stand-up bass on loan and would dearly love to play it.

I'm a left-handed guitar player, with long "fingerstyle" nails.

I've played both lefty and right-handed bass guitars, the latter upside-down, but have had to use a plectrum as the strings damage my nails.

Might it be better to go against my first instincts and learn this thing right handed using my left for fretting?

Please DON'T go into the old "why don't you learn guitar right handed?" routine, that's been visited too many times before.