Hi Dat,
in my past project we created a new attribute for 0GL_ACCOUNT in ecc then bring it into BW, the new attribute will tell if the account is A/L or I/E, then append the letter to that account, in BW we download the hierarchy, modify the hierarchy with the new ID (with a ABAP program), then upload back into BW, then load into BPC.
There are few options as you listed, involve the business and see which option is the best fits your project, keep in mind that maybe one day you want to use BPC data in BW, then you need to find a way to match the GL account.
Andy