It looks like this is already the case, but only documented in the code (lib/galaxy/webapps/galaxy/api/workflows.py):
Update ``step`` parameters based on the user-provided ``param_map`` dict.
``param_map`` should be structured as follows::
PARAM_MAP = {STEP_ID: PARAM_DICT, ...}
PARAM_DICT = {NAME: VALUE, ...}
For backwards compatibility, the following (deprecated) format is
also supported for ``param_map``::
PARAM_MAP = {TOOL_ID: PARAM_DICT, ...}
in which case PARAM_DICT affects all steps with the given tool id.
If both by-tool-id and by-step-id specifications are used, the
latter takes precedence.
Finally (again, for backwards compatibility), PARAM_DICT can also
be specified as::
PARAM_DICT = {'param': NAME, 'value': VALUE}
Note that this format allows only one parameter to be set per step.