I've done some work volunteering with the USBH challenge, and I would have to say it's pretty unlikely you'll find anything more, unfortunately.
First off, records of the underground railroad are basically non-existent, since anybody involved could be prosecuted and punishment was not light.
Slaves were not listed in the census. There were slave records, but that only recorded the age and sex of the slaves an owner had. Former slaves were first listed by name in the 1870 census.
You might get lucky and see them mentioned in a will or other legal document, but I don't have experience with that. You would have to know the owner's name first.
Also keep in mind that, by law, slaves were illiterate. Many seem to have only a rough idea of how old they were, so in a census, their age will often vary by -5 to +2 years from what you expect (natural to estimate more towards the young side). I recently added a woman born in 1880 (1 month old in the 1880 census, pretty sure that's accurate). Out of 6 sources, there were 4 different birth years.