Mark - Thanks for your intelligent reply. Learning quickly about "ecophysiology in lichen secondary metabolites" sure is the strange and sometimes exciting world in which we generalists live.
I was leaning towards not charging also. But the client's program is under development; its last incarnation ran out of funding (I know, not a good show for institutional sustainability/planning) and this seminar could give me some ideas that might contribute to the program's formation or strengths, so I was looking for a way to suggest folding this into my proposal to my client. (I will admit that my time today/tomorrow is being consumed by pro-bono work for another new client and I'm feeling like I give a bit too much away....).