LR does interpret camera-embedded WB numbers but it is limited to 2000K on the red end. Is your custom WB below 2000K?
The way around LR not being able to go red enough for IR is to make a custom camera profile with the DNG Profile Editor that includes a negative WB calibration shift, which can further subtract from the normal limit of 2000K. That still won’t help translate a camera number to a LR number, I don’t think, but it would help you apply a LR-computed number to your IR images, normally by eyedroppering your Custom WB raw in LR with the IR profile place.
Tutorial 4 on page 8 of the DNG Profile Editor documentation describes what to do for IR photos:
The DNG Profile Editor itself is available in the Photoshop Resources list:
http://www.adobe.com/products/photoshop/extend.displayTab2.html#resour ces
