Lower the number of hours. Make it a weekly or monthly trophy for the 'best times' (whether that's a hat or whatever) like with the bk/pk stuff that got leaderboards recently...
Maybe make it so that after a set amount of time holding one tower (1 hour?) the guild has to actively pursue a different tower in order to continue gaining time instead of just camping out? Maybe have 1 hour accessable from each tower each day?
Basically I feel make it a little less of a 'long haul' activity... and more engaging to a larger player base rather than just a few no-lifers...