Well, you can get enmity+ on merlinic via DM augs <,<; If you wanna burn the inventory on that.
But, I can tell you right now... Dispelga has virtually no CE/VE. It's 1 CE 160 VE. And that VE will drain to nothing in 2.6~ seconds. Basically, by the time you can cast it again.. it's all gone, except for that 1 CE. And a GEO's limited enmity set isn't gonna improve that by much. It's fine for getting on the hatelist to begin with, but... It's not doing much to build actual hate. Horde lullaby is 20 CE base. You're not gonna keep hate off the BRD with dispelga if they slept the mobs, unless you wanna cast that 20 times before they leave. And if the BRD tagged any of those mobs before aggroing to pull them, then you're looking at a 200 CE pull bonus you'd need to overcome.
The only way you're keeping hate with dispelga is if the BRD was only aggro'd to begin with. Although... I suppose,
technically, if your enmity set can get that VE to last a bit over 3 seconds, then constant back to back spam could keep the mobs on you vs a lullaby's CE. But.. Dispelga costs a lot of MP. SO.. it's hard to imagine that degree of spam being sustainable.
Unless the specific mobs are dark sleep resistant, I think it'd make a lot more sense to GEO/BLM, and sleepga the mobs the BRD aggro's. That or you'd need to sub something with something actual enmity tools, like /BLU. A single aoe blu spell would rip hate right off a pre-aggro tagged mob and a lullaby even without any enmity+.
Aoe Tagging with dispelga then using a self targeted enmity action is also valid, but... Can't think of many options there for the self targeted enmity action. There's cures... but... you'd need to rig up a cure cheat, since you actually need to cure for HP value to generate the enmity.
And cure enmity is divided by the number of mobs on your hatelist... so if you've got a hefty pull, then you get a tiny pittance of CE/VE per cast. Furthermore, most subjobs that give you cure... also give you Tranquil heart, which reduces cure enmity(/PLD being the exception) -_-; If the pull was fairly small... it might still work for a low enmity situation like brd agro only pulling then sleeping. Still better than dispelga spam though, and more MP efficient.
If you're ML45 and go /run then you could use foil, and that'd work great! rofl. XD