There are quite a few online references to policy and procedures for the researchers conducting clinical trials with human subjects. Most deal with the taxation paperwork required to satisfy the IRS (such as 35% witholding) and do not touch on visa status. There is also a wide variation in policies, indicating that this is more than a bit of grey area, such as a "recent" reclassification of human subjects as not independent contractors.
The closest I could find to answering your question is this one from Vanderbilt University that states that non-Vanderbilt-sponsored H1-B participants cannot receive remuneration but can participate.
International persons (business visitors) receiving payments for
compensation as human subject participants are required to complete
the online Business Visitor Questionnaire (BVQ) and allow the ITO to
create an international tax record before payment will be issued. The
online BVQ can be found here. Note again that persons in H-1B status
who have not been sponsored by Vanderbilt University, cannot receive
compensation for human subject participation. These persons may
participate in clinical or research studies but no compensation can be
provided as this violates the terms of their H-1B visa status.
Vanderbilt-sponsored H-1B employees, however, can receive compensation
since they hold an employment relationship with Vanderbilt.
Probably, declining payment would be the prudent course if they decide to participate. Maybe request that it be donated to a food bank or other charity in the name of the University.