You get some from HK's but its very very slow, i think I was getting 1 guild rep every 10-20 HK's with the 100% guild rep tabard on.
Exalted is possible but it would take months/years of hard farming.
EDIT: I farmed up 218 guild rep from aprox 3000 HK's (for the northrend reps in 4.1) with the 100% guild rep tabard., that brings us to roughly 15,000 HK's per 1000 guild rep, neutral -> exalted is something like 43,000 rep so you're looking at somewhere in the region of 645,000 HK's to get exalted with your guild.
That was killing a level 20 char tho, I believe killing a level 85 gives roughly double the honor I was getting so that would cut the required HK's in half, but would also bring with it other problems.