I may not be reading this clearly or understanding it but this article leads me to believe you would need to replace the stem for a center pull brake to work. Just quoting from http://www.slowtwitch.com/...ams_its_IA_5210.html
As far as I can tell this is THE stem for this bike no other sizes or pitches (yet). This is a good choice if you're just going to make one stem. Plus, you can stick any standard stem on here you want, though it might not be the cosmetic match that Felt's Tri155 stem gives you. Plus, some stems of this sort are beveled underneath, acting like a headset top cap. I don't know if this is the case with this stem but, if it is, that might make any replacement stem look a little funky if there's a space between the head parts and the bottom of the stem. (When Felt answers my question about its stem I'll add an addendum at the bottom of this article.)
A little bit of me wants to see this bike with a TriRig Sigma stem and an Omega center pull brake. Here's a thought. If I'm Felt I drill a hole with a ferrule stop right in my Tri155 stem, just as TriRig does with its Sigma stem. Then if a customer wants to buy a center pull brake for this bike TriRig, Magura, whatever I only have to buy the front brake caliper and not a new stem.