I've been playing with custom boot images for the P7 (I run Ubuntu in a VM on my machine). Unpacking, repacking is not a problem. But I've discovered that "fastboot boot myboot.img" doesn't work! The error message is "remote: command not allowed". I know I can flash my custom boot image that is "fastboot flash boot myboot.img" certainly works. I've only done this once as my phone fell over and I had to rebuild it. Time consuming as you know. Is this a special feature of Huawei phones designed to make life interesting? Is there a work around/solution?
I've found much to my dismay that Huaweis implementation of the bootloader is highly constrained. Most of the normally available fastboot commands have been turned off. Very disapointing. No easy way to test if a custom boot image works without flashing. And then if it doesn't, one has to rebuild from scratch or re-install from a nand backup.