I should think that with this option you should perhaps bill for it an
add a reasonable uplift to the cost. What is reasonable? I have been
working as a contractor for three years (in another field) but the
agency doubles my hourly wage and passes the full cost on to the
customer.

Alternative two - compile a list of local researchers who will do
research
at your facility for a fee to assist those who cannot come themselves.
Send
the list to those sending you research requests.