I tried this on up to date reosaveok's sandbox.

By using URL similar to http://musicbrainz.org/cdtoc/_9LpoD6nJFTW._L_QU3aytlt_LI-/set-durations?medium=1127603 and changing the last number (medium ID) to any valid medium ID (medium format doesn't have to be CD or (unknown), I managed to do it with 12″ vinyl), it is possible to change the track durations of that medium, as long as track count of that medium is not less than cdtoc track count. No warning no error is given. The edit passes without fail.

Also noteworthy: when trying to change track durations on a medium with less tracks than cdtoc track count, there were warnings as well, and I was able to make an edit. But when trying to apply it, I did face the following ISE:

Caught exception in MusicBrainz::Server::Controller::Test->accept_edit "Can't call method "id" on an undefined value at lib/MusicBrainz/Server/Data/Medium.pm line 199.
at lib/Sql.pm line 320"

Stack trace:

line 43 MusicBrainz::Server::Controller::Test
line 24 MusicBrainz::Server::Controller::Test
line 326 MusicBrainz::Server
line 300 MusicBrainz::Server
line 327 MusicBrainz::Server
line 341 MusicBrainz::Server
line 386 MusicBrainz::Server

Request data:

$VAR1 = {
'query_parameters' => {},
'body_parameters' => {}
};

Host: rika

Interface language: en