I was looking at the schematics and saw that there's a SLAM switch as well as a coin switch for left, center and right, so let's see if we can hook that IPT_COIN3 and IPT_TILT up:

[Linked Image from i.imgur.com]

Code
#define BZONEIN0\
        PORT_START("IN0")\
        PORT_BIT( 0x01, IP_ACTIVE_LOW, IPT_COIN1 )\
        PORT_BIT( 0x02, IP_ACTIVE_LOW, IPT_COIN2 )\
        PORT_BIT( 0x04, IP_ACTIVE_LOW, IPT_COIN3 )\
        PORT_BIT( 0x08, IP_ACTIVE_LOW, IPT_TILT )\
        PORT_SERVICE( 0x10, IP_ACTIVE_LOW )\
        PORT_BIT( 0x20, IP_ACTIVE_LOW, IPT_SERVICE1 ) PORT_NAME("Diagnostic Step") \
        /* bit 6 is the VG HALT bit. We set it to "low" */\
        /* per default (busy vector processor). */\
        PORT_BIT( 0x40, IP_ACTIVE_HIGH, IPT_CUSTOM ) PORT_READ_LINE_DEVICE_MEMBER("avg", avg_bzone_device, done_r)\
        /* bit 7 is tied to a 3kHz clock */\
        PORT_BIT( 0x80, IP_ACTIVE_HIGH, IPT_CUSTOM ) PORT_READ_LINE_MEMBER(bzone_state, clock_r)


void bzone_state::bzone_coin_counter_w(offs_t offset, uint8_t data)
{
//      machine().bookkeeping().coin_counter_w(offset,data);
        for (int i=0; i<3; i++) machine().bookkeeping().coin_counter_w(i,BIT(data,i));
}


And now coin B and C will get incremented:

(if you want to see it increment, bring up the bookkeeping, take it off with TAB, then hit 567 and hit TAB again to bring back the bookkeeping. Do it fast enough and you'll see the number increment)

Interestingly, hitting 567 simultaneously doesn't register, but 567 quickly in sequence registers.

Hitting the T for TILT causes the game to screech and ignore the coin input. (No hitting the coinbox to get free plays!)

[Linked Image from i.imgur.com]