This is a spin-off of the Wobble Arrester by twotimes:
http://www.thingiverse.com/thing:2151
I love his idea, but I had a hard time getting the offset linear guides to work properly - my prints always had a sort of blobby section right at the top, which took my bushings out of alignment and caused binding. No doubt I still need to work on my skeinforge skills.

Anyway, I figured that ideally the Z-rods should be somewhere in the middle of the Z-stage - in other words, between the drive screws. This should reduce binding and in the event of a crash won't load the Z-rods and bushing clamps as badly. It also turns out that there's space enough to add the standard makerbot rod mounting scheme: through holes with external covers, although the covers are a little different due to space constraints. I have uploaded DXFs that can be used as templates for modifying your top, middle, and Z-stage; if you have access to a lasercutter you could use them to cut drop-in replacements to the standard makerbot parts.

I certainly don't want to step on any toes, but I'd like to see Z-rods as a standard option, so I tried to make this mounting arrangement easy to implement on the production side. Unfortunately I couldn\'t find a stock part for the bearing mount, but it should be easy to machine in quantity if it comes to that.

The bearing retainer comes in 3 flavors: 3/8" through hole for use with bearing inserts, 1/4" through hole for use without bearings, and 3/8" without counterbore for printing. (Note that if you intend on printing you'll have to use a non-headed bushing or relieve the underside of the Z-stage to accommodate the bushing head.) Just choose your poison.

EDITED TO ADD:
I noticed an error in the modified Z-stage DXF, so I have removed the old file and uploaded a corrected one. The first version would still work fine as a template to modify an existing stage, but not for cutting a new one. Please download the updated version.

1) Alter your existing top, middle, and Z-stage OR cut yourself a new set.
2) Make a pair of bearing mounts - I'd suggest delrin or aluminum for the 3/8" counterbored version and delrin for the 1/4" version. If you print them, be ready to tap the 4 M3 holes for assembly.
3) Make 4 rod retaining covers either by hand or on a lasercutter
4) Print 4 Z-floats from the aformentioned Wobble Arrester by twotimes (or your preferred substitute).
5) Install and lubricate.
6) Get back to work, lazybones!