As it happens, I've been playing around with that concept in my current campaign, which is a Birthright variant mixed with some PFRPG, BECM, and other bits.
I'm using the Landlord feat to represent level based boosts to kingdom income, giving out various Leadership and organization feats for free, and allowing the PCs to use their followers as a pool for die modifiers to kingdom actions.
My purpose with this question is, how do I figure out the cost of a military unit of warforged, from the creation forge, to the PCs.
Likewise, what if they want some cavalry (chargers) or heavy siege/naval/air units (which are actually non-sentient, so don't have the "slave soldier" concept.
Further, given the alignment attitude of this particular group, they wouldn't take on the warforged as slaves anyway, so it is really an issue of how much should they cost to produce.
The undead question is because I like to be able to justify, within the rules, how the bad guys do stuff, rather than just handwaving it. (I may feel like sending a horde of undead against my PCs. Thus they may want to have some warforged around to kill the dead things and avoid having their losses "defect" to the enemy.)