There is no Lightroom release-candidate to directly work with D750 raw files, but the DNG Converter 8.7 RC is on Adobe Labs: http://labs.adobe.com/ and can be used to make DNGs from the NEFs that will work in LR 5.6. The one issue would be that the camera profiles other than Adobe Standard won't be accessible, unless you manually copy them over to your Users profile area.
The link to the DNG Converter 8.7 RC download page on the CS6 technology page is currently mistyped and needs to be this: