For this issue tonemapping is a good start, still there are some options/alternatives to keep in mind:
1. add a mantle to those spikes: copy the spikes emitting light and displace along normals by eg. 1-100um, same material as original object. Then you can just apply a mix with null material and a blend texture to mix both materials in a way that the light parts can be seen. To change the color you might use also absorption.
2. like 1, except you dont create a second material but directly blend the material with emit to the material without emit (everythign else the same).
3. tonemapping: tonemapping alone with LuxRender might not do the job, since LuxRender tonemapping is global aka it effects the whole picture. In this case by tuning the light, the whole picture becomes darker. Instead Id suggest using qtpfsgui and apply Reinhard '02 tonemapper, which is a local tonemapper and computes tonemapping for areas of a picture (instead of reinhard '05 in LuxRender).
4. blackbody: create a blackbody material preferably at app. 3500°K and mix with the non-emit-material
Just my five cents on your options which might deliver varying interesting results