Why it is not possible to create custom AM which behaves similar to btree?

Hi hackers.

I tried to create LSM AM which can be used instead of nbtree.
I looked at contrib/btree/gin, contrib/isn and try to do the following:

CREATE OPERATOR FAMILY lsm3_float_ops USING lsm3;

CREATE OPERATOR CLASS float4_ops DEFAULT
    FOR TYPE float4 USING lsm3 FAMILY lsm3_float_ops AS
    OPERATOR 1  <,
    OPERATOR 2  <=,
    OPERATOR 3  =,
    OPERATOR 4  >=,
    OPERATOR 5  >,
    FUNCTION 1  btfloat4cmp(float4,float4);

CREATE OPERATOR CLASS float8_ops DEFAULT
    FOR TYPE float8 USING lsm3 FAMILY lsm3_float_ops AS
    OPERATOR 1  <,
    OPERATOR 2  <=,
    OPERATOR 3  =,
    OPERATOR 4  >=,
    OPERATOR 5  >,
    FUNCTION 1  btfloat8cmp(float8,float8);

ALTER OPERATOR FAMILY lsm3_float_ops USING lsm3 ADD
    OPERATOR 1  < (float4,float8),
    OPERATOR 1  < (float8,float4),

    OPERATOR 2  <= (float4,float8),
    OPERATOR 2  <= (float8,float4),

    OPERATOR 3  = (float4,float8),
    OPERATOR 3  = (float8,float4),

    OPERATOR 4  >= (float4,float8),
    OPERATOR 4  >= (float8,float4),

    OPERATOR 5  > (float4,float8),
    OPERATOR 5  > (float8,float4),

    FUNCTION 1  btfloat48cmp(float4,float8),
    FUNCTION 1  btfloat84cmp(float8,float4);

But then I get error for btfloat48cmp and btfloat84cmp functions:

ERROR:  associated data types must be specified for index support function

If I replace lsm3 with btree in ALTER FAMILY, then there is no error.
I wonder if it is possible in Postgres to define custom index, which can
handle comparison of different types, i.e.

create table t(pk bigint);
create index on t using lsm3(pk);
select * from t where pk=1;

I failed to make Postgres use index in this case. Index is used only if
I rewrite this query in this way:
select * from t where pk=1::bigint;

Thanks in advance,
Konstantin