key   val
------------
0     -999
1     100

DROP TABLE IF EXISTS lookup;
CREATE TEMP TABLE lookup
AS
SELECT *
FROM (
   VALUES (0,-99999::numeric), (1,100::numeric)
) AS t (key, val);

UPDATE my_rast SET rast =
   ST_Reclass(rast,
       ROW(1, (SELECT string_agg(concat('[',key,'-',key,']:', val), ',') FROM lookup), '16BSI', 0)::reclassarg)

UPDATE bin_rast SET reclass_rast = ST_Reclass(bin_rast, 1, '0:-999, 1:100', '8BUI', 0)
WHERE rid = 1;

SELECT string_agg(concat(key,':', val)::text, ',') FROM lookup;

UPDATE bin_rast SET reclass_rast =
   ST_Reclass(bin_rast, 1,
    (select concat('''',string_agg(concat(key,':', val)::text, ','), '''')
     FROM lookup)::regclassexpr,
    '8BUI', 0)
WHERE rid = 1;