I don't think that is possible through survey123 itself. However, you could add a hidden field (yes,no) have it default set to no, Have your value question in the read-only column set to ${<helper_field>}='yes'. Then write a python script to post process the helper field based on the entry.
Something like this to get you started (I haven't tested it so it may need adjusting but should be close):
from arcgis.gis import GIS
portalURL = "https://www.arcgis.com"
username = "USERNAME"
password = "PASSWORD"
feature_item_id = "<ItemID>"
gis = GIS(portalURL, username, password)
feature_item = gis.content.get(feature_item_id)
layers = feature_item.layers + feature_item.tables
for layer in layers:
features = layer.query(where="1=1", order_by_fields='objectid ASC').features
for feat in features:
gid = feat.get_value('globalid')
value = feat.get_value('value')
value_length = len(value) if value is not None else 0
if value_length > 0:
where_clause = f"globalid='{gid}'"
layer.calculate(where=where_clause, calc_expression={"field": 'helperfield', "value": 'yes'})
This will loop through all layers and tables, you can adjust to a specific one if needed.
edit: Changed this line: value_length = len(value) to this: value_length = len(value) if value is not None else 0
