Hi sorry been a few weeks since I had a chance to be on here and look around. What I am doing is similar to your approach but made dc-dc boost converter making a homemade battery charger that uses say 8-10 kw hr old school FLA group 27 12v and exports 375-380 VDC to i3 battery pack by splicing into main HV cable between inverter and pack.
Will be testing next week , but using the wisdom as REX wont turn on until 25 % discharge to prevent over-charging , And like in Nissan leaf homemade REX just need to turn on BMW first put in drive mode and then use my exported power when down to say 45 miles range left on my Guess-ometer. and then turn on my added contactors and put a blocking diode in with HV V+ on my Aux Rex, so if it goes into regen mode it wont back feed into my aux battery REX if regens while I'm Aux REX charging. Then when get to my destination , simply de-contact my aux battery REX before the i3 is turned off and main battery in EV de-contacts
So I am going to charge and drive hopefully like it should of always been.
I made a park and charge aux battery kit that fits in my 2013 Ford Focus Electric. Here is a local news story I live in rural Iowa and its 40 miles to any Level 2 Public Charging and only about 8 in a city of 200,000. http://www.cbs2iowa.com/news/local/recharging-your-ride
Hope this is ok, about the homemade invention at that time not an offer for sale.
I am sure its going to throw a million codes and then I have a freind that can do a work around. I am sure though there are built in BMS safe guards that will shut down EV to prevent overcharging.
Maverick Inventor -- Who has an interest in getting our world off of Oil based fuels -- to EV Use
2009 ZENN Low Speed Vehicle 72 Volt GEM Controller
2013 Ford Focus E-Hatch
2014 BMW i3 with REX 60 AH Battery