What's the problem loosing some adresses at the low or high end of the adress range?

But the SJA1000 has 12 registers and a multiplexed adress/data bus, right?
I only had a glimpse of the block diagram some days ago but a simple 7402 should be enough as a decoder if you use ALE of your micro, one select pin and adress the CAN thing in the Intel mode. Than you should be able to write to it in the way:

mov A,#data ; load byte
mov R0,#reg_no ; load register number
clr Pxx ; enable chip
movx @R0,A ; write byte into chip
setb Pxx ; disable chip

Check the timing of the SJA1000.