Skip BFG, and go straight for SGMiner. It's built and designed for scrypt GPU mining, and already has this feature added. Best part is you don't have to build it, as there are already pre-built versions floating around. Just check the official LTC forum for download links.
It is not "built and designed for scrypt GPU mining" any more than cgminer or bfgminer. It's a copy of them with ASIC support removed and a handful of optimizations added. The noteable ones have already been added to bfgminer.
If you want a scrypt-only, GPU-only miner (for some reason) go for it.
Edit: In fact, if you look at the SGMiner commits, they are pulling in code from BFGMiner including Luke's recent OpenCL fixes.
Edit #2: I have nothing against SGMiner btw. I just don't personally see a reason for a GPU-only, Scrypt only miner (SGMiner) just like I don't personally see a reason for a ASIC-only, SHA only miner (CGMiner). Especially if there aren't tangible benefits. I also don't think it's particularly productive to create yet
another fork of a miner when there are already several in active development.