I have been doing a little more reading on the Message-boards here and I now know that this is, most definitely, not an "out-of-the-blue" question. It seams that many a gamer is in the same mind frame as myself. So, I have a few ideas to accomplish this book;
The SBG is not a long book. Only 128pgs. If a comparable book where to be made it could, very easily be made a part of a more comprehensible book such as a Kingdom Builders Manual or a Book of World Design. (or something) In a KBM you could certainly have an entire section devoted to sieges and other Wargames, including warmachines and large scale battles. All rules that wouldn't "have" to be used unless a particular campaign asked for it. The economics for a kingdom/war could be simplified to something usable in a game setting without too much fuss.
Other sections could include comprehensive rules for building kingdoms in various terrain and trade costs, in and out, for natural resources and and goods.
But back to the SBG part, the whole concept could be toyed with but the gist of the original book could still be used, i.e. lists with prices of components and modifiers for different conditions could still be used and not infringe on the closed source material from the original book, I think. Also a more understanding implementation of Character available resources, such as magic, would be nice. Also the clear cut 30% of cost is labor is pretty asinine, it should be divided into materials and then labor. That way you could buy more workers to finish things faster or it could account for the PC's doing the labor themselves. Perhaps something along the lines of; it takes a 1 builder 1 week to build 1 stronghold space, it takes 2 half, 3 a third, etc. to a minimum of 1 day. You could pay more also to decease the time. PC's may roll craft skills to decrease time.
There is of course MUCH more that would go into this book but I would like to hear from other people what they would like to see in it too.
-Timber