There's a pretty big chicken and egg problem with this, in that to make it useful you need a lot of users in each travel destination to make this work. If you're active on CouchSurfing, I would try to find locals in popular travel locations who would be interested in giving it a try, or maybe work out referrals with hostels to promote your site locally in exchange for promoting their hostels on your site. For example, for every 100 users they refer, they get 1 week of free advertising. You might even work with Hostel World to drive referral traffic to them with the hostels you've made agreements with in exchange for some free advertising on their site in a similar manner.
Given your demographic I'd also post in University/ College sites. When it comes to hostels/ bed and breakfasts, you can probably contact some of the industry organizations to get lists and partnership or advertising opportunities - and of course call up hostels in some of the bigger cities to have them put up a notice for you. Get a list of journalists that write for travel sites and magazines and send them an intro email - maybe you'll get lucky and be picked up by one or two, offer an exclusive interview if they pick it up ( and stick with your promise).
All of this is legwork, but (except for the advertising) it's pretty cheap to do - just a lot of salesmanship - which is laborious but I figure you have more enthusiasm than money.
Hope that helps - good luck!