No need to relock the bootloader. Just flash stock recovery and you're good to go. Now, that being said, I don't know if an OTA update will relock the bootloader itself, if it does that's an easy fix.
You just need to have stock recovery for the OTA update to install properly.
EDIT: Just re-read the last part of your message... To keep stock ROM, but have 4.3 you only have a couple options. 1) Go through the process of gaining S-Off to be able to flash the Google Play Edition RUU. Once done properly, this will allow you to receive OTA updates just as a "normal" GPe device does from Google. 2) Find a ROM that is based on 4.3, but is running the Sense 5.0 overlay; keeping your device stock looking, but with the advantages of 4.3 and a custom ROM.
Maybe check out Viperboy's ViperROM? He designed the software to pretty much look exactly like stock, but with many cool features added in. I've not flashed it myself, but I've read a lot of good things about it. I'm not entirely certain if he's based it off of 4.3 or 4.2.
Hope this has helped some!
Posted via the AC App from my N7v2 or HTC One