The good news is the B5 ROM itself is very solid and accommodates a wide variety of KitKat based kernels. AFAIK the only major bug left is Bluetooth and there are workarounds for most everything else. Hopefully the developer tools like APKtool and the Xposed Framework get updated for L soon and we can begin getting some of the "nice to haves" that users have been asking about.
By re-basing it on the stock L configurations all your enhancements should be more easily moved to the release version of L, and to the subsequent updates that are likely to come pretty quickly based on the history of Jelly Bean and KitKat.
And if you are looking to produce generic tools and/or environments, they have to be compatible with the stock L ROM's for other devices otherwise you will wind up completing with CM, AOKP, Paranoid Android, Slim and other ROM's whose cores are enhanced versions of the AOSP source. IMO, if you want to build an "AOSP enhanced" version of the stock ROM's you are better off starting with the stock version of those ROM's. That does mean you will never be able to compile the complete ROM from source, like the AOSP enhanced ROM versions, but even those still have to depend on some proprietary binaries.
FWIW, that's the way I see it.
ramjet73