I wanted to experiment with this and found that read-only access 
(darcs clpone, darcs pull) works just fine, but darcs push (with write 
access) does not. I investigated and found that when we identify the 
remote repo we issue an scp command. Is it possible to use transfer-
mode instead?

I made a stupid mistake in the access file. Works now.

I found a bug in the script though: when it checks the access rights 
for the transfer-mode command, it should accept both "r" and "rw", but 
it only accepts "r". Will send a fix.