Hi Lucas,
Sad to tell you, Illuminator services have specific modes and parameters that could be passed to it. What you can do is get all the users in an XML and then filter based on /Row[User="Lucas"].
I guess you would have gone through this link, still for your reference, below are the services available:
Illuminator Services of SAP MII
Also, check for a work aroung suggested by Christian in below thread:
MII Admin Service - All user attributes (teleph... | SCN
Hope it helps.
Best Regards,
Swaroop