Is there an equivalent way to do MySQL's JSONOBJECTAGG() in MariaDB?

No, MariaDB still does not support JSON_ARRAYAGG and JSON_OBJECTAGG functions. A JIRA ticket has been raised for requesting this feature: https://jira.mariadb.org/browse/MDEV-16620

Now, from the docs of JSON_OBJECTAGG():

It takes only two column names or expressions as arguments, the first of these being used as a key and the second as a value.

An error occurs if any key name is NULL or the number of arguments is not equal to 2.

However, you are specifying three arguments in JSON_OBJECTAGG(awards.fiscal_year, coapplicants.coapplicant_name, coapplicants.organization_number); so your attempted query will not work as well.

Now, in the absence of the required functions, we can utilize Group_Concat() with Concat(). I am assuming that you need only first two arguments (as explained in previous para).

GROUP_CONCAT( DISTINCT CONCAT('"', awards.fiscal_year, '": "',                              coapplicants.coapplicant_name, '"')               SEPARATOR ', ')

Note that, in case of string getting very very long, Group_Concat() may truncate it. So, you can increase the allowed length, by executing the following query, before the above query:

SET SESSION group_concat_max_len = @@max_allowed_packet;