As it stands today the ARI will sort and present ALL names in the voicemail.conf file for forwarding to (in the forward to dropdown list) ignoring the vmcontext. But the voicemail system when you dial it will only allow you to forward voicemails within the same voicemail context of the account you are currently using. So if I create a special group called restricted and via the phone I use one of those extensions I can only forward a call to another restricted context extension. The ARI should work the same way to be consistant.
There was a patch to do this posted on the site back on June 11th that fixes this issue. see http://www.freepbx.org/forum/changes-to-the-forward-to-button-in-ari the 2008 postings at the bottom.
it's a simple 1 line insert and I can confirm it works as it should.