Wow! What a quick set of responses! Thank you all for your suggestions - I will be 
checking them out (slowly...) over the summer - I'm actually still working full-time, so 
my birding is restricted to weekends or days I can afford to play hookey (like the 
Friday afternoon last year that I skived out early to go up and see the Lapwing...).

I'll periodically re-post this message to keep collecting possible locations for the list - 
as you all visit your favorite birding spots, make a mental note about parking, trail 
steepness, large obstacles like car gates with little teeny footpaths around them (as 
is the problem at Hughes Hollow) and, of course, bathrooms!

There is a fledgling US chapter of the Disabled Birders UK organization, but their 
website hasn't been updated since 2001 or so, so I don't think they're very active. 
Again, thanks for the quick responses!

Mary