Why do you want Bv in these units ? If you are doing any sort of calculation on them, you'd be much better off just working in native APEX96 units.
Edit: Or APEX, if you aren't doing the calculation on the camera.
Edit:
Looking at doug kerrs apex doc
http://dougkerr.net/pumpkin/#APEX it seems like you should be able to approximate it in lua something like this
luminance_x10 = bit_shl(1, get_bv96()/96) * 34
You'd have to be a bit more sophisticated if you wanted fractional stops, but at that point a very rough approximation should be fine.
Note that Kerr's definition appears to disagree with what shooting_get_luminance is doing
Kerr defines Bv (with L in candelas/m^2) as
Bv = log
2(L/0.3K)
=>
L = 0.3K*2
BvWhich matches the table on page 5.
shooting_get_luminance appears to be doing
L = 100 * k * 2
(bv96-168)/96 with K = 12.5 rather than the 11.4 Kerr uses, the 0.3 factor dropped, and an unexplained - 168 (=1 3/4 stops) subtracted from Bv ?