Im not from that area but what I would do is see if theres any local forums. You can keep mentioning you are in the area and the local punters will often know of discreet hotels if you get friendly with them. Also change your location on Adultwork ahead of time. You will get bookings no doubt, its just a case of people knowing you are there. You could also try placing an ad in the Daily Sport but the day before as blokes often dont get around to browsing the small ads in the staff room on thier lunch break until the next day. Its about ?30 for 2 lines.
Good luck and let us know how you went on as I wouldnt mind a trip to York if I get chance this year.

There isn't a North Yorkshire forum, but there is the West Yorkshire Sex Guide (think there's a link on main page) and also Hull Adult Forum which are smallish boards, but do mention York now and again. Leeds is in West Yorkshire and pretty much at maximum density prossie-wise but the novelty factor of being only there briefly should bring in some bookings; the Punternet tour board is another good place to advertise in this part of the country if you already have a paying ad.

If you are talking specifically about North Yorkshire (York, Harrogate, Scarborough and so on) PM me - I can help with hotels and whatnot if you like  . Get through to the coast while the weather lasts!