You don't need a bar unless you're doing weighted pullups-- even then it's not mandatory, only helpful. If I'm in a gym without a bar I'll pullup off anything possible. The mixed and awkward hand positions improve both grip as well as the primary muscles themselves --like mixed grip pullups do. I've used smiths, power racks, the top bar holding the pulley on lat pulldown machines, the bar itself on the pulldown machine (with the weightstack set above my BW), and all sorts of other crap. For stuff with room for only one hand you can try one arm eccentrics, depending on your strength. Seriously, just monkey up random shit and you'll be set.