Looking at Facegirl.ch, it seems there are few (actually very few) girls that could be contacted by e-mail, but that seems to be the site's own message system, and you need to register there.
On the other hand, you can pretty much walk up to any of the Geneva-Girls locations (all places are clearly displayed on their site) and see who's free and what's on offer, or go there and book a meeting for later if said girl is busy. I suppose the same goes for Venusia. I think you have to book when it comes to CindysGirls and ParadiseGeneve.
Or as said above, you might just walk to the Paquis neighbourhood, next to the station, and check on streetwalkers and surrounding clubs and parlours.
I'm not sure if there's a "salon" that can arrange a meeting through mail. Venusia and Paradise have some messaging available on their website, but I don't know if they'll eventually ask for phone/SMS communication.
Still, if you've got a lot of money to spend, Easylives and Agence-Elegance have e-mail addresses. Though at 700 francs / 1 hour, I fail to see the point of going through them. Granted, Easylives is rumored to have had Jade Laroche occasionally working under the radar for them, some time ago, but except for such high-profile girls, that's quite a ludicrous price.