Looking through the code, it seems the game is using the method
GetChannelName to determine what guild name to show. It uses the information in
chatdata.lua to that end. You could try to overwrite the channel info so it no longer uses a dynamic name for the guild channels and then set your custom name like this:
Lua Code:
local ChannelInfo = ZO_ChatSystem_GetChannelInfo()
local channelId = CHAT_CHANNEL_GUILD_1
ChannelInfo[channelId].dynamicName = false
ChannelInfo[channelId].name = "MyCustomGuildName"