Sometimes "low bono" appears to be used as an adverb, as in the sentence "I was working this case low bono." or "I am working low bono;" but this is probably a shortening of "working on a low bono basis," rather than a truly adverbial use.
"Solos, Small Firms Team Up to Support 'Low Bono' Legal Work" Small Firm Business 03-31-2005 .
"Law Schools Urge Graduates to Start Small and Think Local" New York Times 12-16-2002 
"The Changing Face of the Legal Profession: Using a Client-Centered Approach to the Provision of Legal Services" by Elizabeth Gamsky Rich in Think Small, The Official Newsletter of the Solo and Small Firm Practice Committee, The State Bar of Wisconsin, Volume 1, Spring 2004, page 3