MR was very rarely used for its intended purpose (guild wars). Mostly it was abused as a cheap tool to bypass nearly all of the limitations imposed on the PvP system, and thereby avoid the consequences of killing lots of players, whether related to some particular "KOS" or not. Everyone who had the balls went on a killing spree while being red (exception being the "naked" reds who maybe did not have the balls, but they still submit themselves to the limitations by giving up their gear), the irresponsible and the cowards did it while cowering under MR.
If you need a buff for an all-out war with other guild(s), there should be an NPC where the GMs of those guilds can declare a war on another guild (one), which will provide the effect of the MR buff when killing members of that guild. The target guild will accordingly have the same effect in place when killing members of the guild that declared the war. A guild can also declare a war on more than one guild but the 2nd and latter guilds will have to accept the declaration. However any number of guilds can join in by choosing a target guild.
If you need the MR buff to get rid of just one KOS person or party, then your guild is either seriously unmanned or unskilled.