I've toyed with these solutions over the years, the reason they are not implemented by default in lxle is, brightside sucks up to much memory for my liking, and skippyxd is inconsistent and a bit buggy. But using these two solution will provide your hot corner expose feature. :)

BrightSide Properties provides hot corners
http://www.webupd8.org/2011/04/set-up-hot-corners-edge-flipping-in.html

SkippyXD is 'Expose'
https://code.google.com/p/skippy-xd/