Haunt Shift from Libris Mortis can let undead creatures haunt and animate objects up to huge size. Building a heavy duty object in the shape of a mech with interior spaces could work quite well.
JaronK
Hmm, that works, but I was thinking more of a 'mechanika'/steampunk feel than necromantic construct feel. Of course, thinking about this, Iron Kingdoms already has rules for Warjacks.. from there it's a pretty short leap to mecha, even shorter than scaling up the powered armour in that web supplement.
Note that haunt shifted critters could theoretically be sectional... have one critter controlling the huge sized legs and torso, while the other ones control the arms and head and count as riding the legs like a mount?
Or, just have the Haunt Shifted huge contruct wearing clockwork armor. It would be a giant spirit powered (Aether?) statue made of whatever you like (preferably something with a solid hardness score) clad in gears and pistons. It's seriously steampunk/magitech. If you want, get someone who can cast Extract Gift as a spell like ability (Spell Stitched Undead, Runesmith, whatever) and summon demons, then syphon their power into your clockwork death machine. Separately Haunt Shift some medium sized turrets that you mount to your mecha. Planar bind and kill some Orthons (FC1) for their Hellfire Crossbows, use the savage species ritual to give the turrets the Tenari subtype, and now you've got independently firing turret guns that launch bolts of hellfire up to 400ft with unlimited ammunition. What could possibly go wrong?
JaronK
What indeed? You're thinking more magitechy, though, than i'm going. And more gundam/macross style mecha, I guess I should have specified. I'm looking for specifically Large/Huge style mecha, specifically like that thing Ripley uses to fight the alien queen in Aliens. It's magitech in that it's powered by magic, but it's more.. mundane than hauntshifted demongifted multimind mecha. And it looks all steampunkey. It's not for a PC, although they might get their hands on it. It's for the PCs to fight, if they botch a social encounter. Yes, i'm making a mecha-based military in a fantasy setting. Why? Because i'm an evil, horrible little man.
Could also look at the Dragonmech campaign setting. It's all about mecha, including city-sized ones.
I had a quick look at that a while ago, and it seem to be very complex. As in, 'use this for a campaign setting' complex, not 'use this for one encounter' complex. I could be wrong, it was a while ago, though.