First of all, I don't consider myself a remote-sensing nerd.
I didn't realize boreal means mostly coniferous trees.
Ed's idea of lidar intensity makes a lot of sense to me, just based on ground- or all-last-return- intensity images I've seen. Depending on the thickness of the canopy or how close together the trees are, you might have to throw away points away from nadir. (I'm assuming nadir is the best place for the laser to penetrate to the ground.)
The intensity image products I see tend to be grayscale images. One could probably determine the intensity returns of interest and apply filtering or false-colors to highlight them.
I see some search hits for things like "Multi-wavelength canopy LiDAR."
A multispectral lidar that could do something like NVDI and let you only process points near the ground might work.