You can only install custom ROMs based on stock ROM and use stock kernel. You can also head to the development section and find a ROM that runs on a phone with a locked bootloader. The developer will usually tell you if his ROM can run on a phone with locked bootloader.
Microsoft Certified Professional
Sony Ericsson Xperia Arc S(LT18i) - Unlocked BL
2.3.4(.42) - > 2.3.4(.62) - > 4.0.3(.562) - > 4.0.4(.587) - > back to CM10.1(4.2.2) from UHD
Sony PS VITA PCHJ10001(初音ミク Project Diva F Limited Edition)
1.69 - > 1.80 - > 1.81 - > 2.00 - > 2.02 - > 2.11 - > 2.12 -> 2.61
Sony PSP - 2006(Replaced the Mobo which came from a 2001)
3.60 OFW - > 5.00 M33 -> 5.50 Gen D3 -> 6.60 PRO B10 - > 6.60 PRO C