I bricked my Tf300TG too. I can access only APX. Has no sens to change the mainbord only becouse Asus restrict the access on NVFLASH. If we will sign the petition from XDA forum I believe we have chances to get access.
I did tens of changes or upgrade on my Galaxy S2 and I never seen any problems and any restrictions.
I not recommend to anyone to buy Asus tablet.
I'm very frustrated.