Any cyclocross or road-oriented frame with track ends or horizontal dropouts will do. I wouldn't go all out trying to find the slackest frame ever...something with good road-oriented geometry should suit you best.
Off the top of my head...Surly Steamroller, Cross Check, Motobecane Fantom Cross Unu, Redline 925, many BD offerings except the Kilo TT or Motobecane Pro Track. Look at IRO frames as well. There are many others. Just avoid anything described as having track geometry and you should be fine.

For what its worth, I've ridden several 100+ mile road rides on stiff, quick handing track frames, and I don't find them to be more or less comfortable than any of my road, cyclocross, or touring bikes. Having the proper bar height, cockpit length, a comfortable saddle, and the right shoe/pedal combo are more important for long-distance comfort. Geometry is secondary IMO, unless you are carrying large loads or riding on rough or technically challenging terrain..then geometry is far more important.