The lack of consent or debriefing in online research has attracted widespread public distrust. How can designers create systems to earn and maintain public trust in large-scale online research? Procedural theories inform processes that enable individuals to make decisions about their participation. Substantive theories focus on the normative judgments that researchers and participants make about specific studies in context. Informed by these theories, we designed Bartleby, a system for debriefing participants and eliciting their views about studies that involved them. We evaluated this system by using it to debrief thousands of participants in a series of observational and experimental studies on Twitter and Reddit. We find that Bartleby addresses procedural concerns by creating new opportunities for study participants to exercise autonomy. We also find that participants use Bartleby to contribute to substantive, value-driven conversations about participant voice and power. We conclude with a critical reflection on the strengths and limitations of reusable software to satisfy values from both procedural and substantive ethical theories.