No.
The reason is this: when you have a locked bootloader, the bootloader checks for signed boot images. If it doesn't detect a signed image, it'll flag a security error. Once the bootloader is unlocked, you can run whatever boot image you want (i.e. flash custom ROMs with custom boot images).
(Some) ROMs can bypass this if a developer is using 2nd-init or hijacking. A good example of this is the bootstrap recovery CWM that is available for L9s with locked BL. Some of the older custom ROMs for the L9 utilized hijacking because they hadn't figured out how to unlock the bootloader. Unfortunately, those ROMs are only for the P760, and are quite old.
On the plus side, since the source is out there, a developer could take the bootstrap code and apply it to a current build of CM10.x for the P769, and have a working build of CM10.x for use with locked BLs.