Okay! Finished working/sandbox-testing, now I'm gonna need a real live guinea pig ;D.
You'll need to:
* Be on 5.2.0
* Be JailBroken
* Not be terribly upset if it fails and you end up on 5.3.0 without a JailBreak

.
What to do:
*
FIRST, Install the update package attached here.
* Then, install the official 5.3.0 update.
If all is well, you should still have the bare minimum after the update (ie. the jailbreak key).
(So far, I only tested this on 5.1.2 & 5.2.0. Another pair of eyes (eureka

) confirmed that it *should* work on 5.3.0, and turning my brain to mush looking at IDA made me pretty sure that the update process wouldn't kill it, so I'm pretty confident it works

).
EDIT: The file (update_jailbreak_bridge_1.7.N_install.bin) has moved to the full 1.7.N release of the
JailBreak 
.