Well according to Lawrence Whitaker (who wrote the Elric MRQ game and playtested BRP), MRQ and BRQ are very close to each other. I confirmed this when I went out and bought RuneQuest Deluxe (I also own the Elric and Hawkmoon games) as well as BRP. BRP to me is kind of like True20, its very good as a toolset. Mind you, I think the two systems work best together. There is a nice conversion system between the two here
Personally I think RuneQuest would be the best system to make a Freeport book for. Considering it already has Glorantha, Elric, Hawkmoon, Slaine, and Lankhmar as settings, Freeport would fit right in. And yes, it is open source, and the MRQ SRD is easily available.
I bought the Pirate's Guide and the True20 companion at the same time, but since the emergence of 4e, I have been veering away from d20 based systems. Mongoose RuneQuest does have that oldschool feel about it.