It's a good idea, just with one minor drawback:
Because the coding in your idea would have to be linked into the information on the battleground server (i.e. whether you did the quest or not would have to be recorded on both servers), there is the possibility for the guild quest (and maybe the guild itself) to suffer the current client-data corruption issues Siege is having. (I'll fully admit it might be the hardware that fried like an egg on a skillet, but given that FEU is not releasing any details about anything at this point, it's everyone's best guess as to whether the coding just flat out broke itself beyond repair or the server was having issues, or both.)
I'd rather just buy the title as we know the cash shop still has some functionality. (And now that I've said it, nobody will be able to buy anything from the CS in a day or two.)