There are lots of aspects to power line right of way access, including the dangers of the lines themselves, who owns the land below the lines (often _not_ the utility), access by 'favorable' users (eg. law abiding users who provide extra eyes, and thus prevent use be criminals...) etc.
I suspect that your club member who posted that it was both extremely dangerous and illegal to off road under power lines simply does not know the full story...if they actually work for the utility, then you probably want to try working _with_ them in order to better define what is safe, legal, and mutually beneficial. From the quote that you posted, this might be a bit of a hard sell education campaign, since they sound excessively scared of electricity (as opposed to rationally aware of the dangers).
Walking under a power line, even a 750kV line, is not dangerous (or at least should not be dangerous if the line is properly functioning and maintained). Climbing the tower _is_ dangerous, and getting too close to the conductors is dangerous; 750kV can arc a surprising long distance. But the design of the power lines (including the height of the towers, the tension on the conductors, the operating temperature, etc.) is supposed to account for this and provide a reasonable clearance for users on the ground. (Note that in the BPA documents above, the design rules for the lines allow vehicles up to 14 feet tall under the lines.)
In any case, there _are_ real dangers, but part of line design is to make sure that 'the public' cannot approach regions of danger without clearly passing some demarcation. In a properly designed power transmission system, you would have to climb a pole or cross a fence or open a locked room or similarly cross a barrier to approach the conductors too closely. Snow mobile users all over the North and ATV users everywhere use power transmission rights of way, safely.
I found one interesting link, which quotes representatives of Northeast Utilities: http://www.inspiredink.com/article.asp?ID=76