It could be nice if it opens in a new window, simple and doesn't stand in the way of the maps in the site.

Might be something like this for the US
http://maps.google.com/maps?hl=en&tab=wl&daddr=DESTINATION-COORDS

or the UK
http://maps.google.co.uk/maps?hl=en&tab=wl&daddr=DESTINATION-COORDS

Example
http://maps.google.com/maps?hl=en&tab=wl&daddr=34%B0%202%2718.66%22N%20118%B016%272.86%22W

You can feed the coordinates to the API and let it figure street addresses.

That seems ok, assuming you have a well established table of equivalence. This might be a start:

http://en.wikipedia.org/wiki/Grade_%28climbing%29#Free_climbing_ratings_comparison_table

Still, UK ratings (ie. E8 6C) are not that straightforward, other than that it seems possible.

And, by the way, I cannot send messages to "Matt" through the bouldr site...

I wrote this one... shows up as anonymous, my username in bouldr is nachotronics