I've just gone down to investigate the Zombie and Skeleton spawners and was slightly disappointed to find that they are just the spawn rooms as found.

Would anybody object if I were to build so kind of mob grinder for them to farm XP?

At the moment we only have two rules for grinders to prevent spawning overload on the server.

1. not within the spawn area of 128 blocks N/E/S/W
2. Factor in a shut off like glowstone and pistons or RS lamps to effectively shut off the spawner.

The second rule is mainly a precaution especially for dark room grinders.

As long as these two rules are followed. go for it dude.