I posted this a little while back, found in the Australian...

I posted this a little while back, found in the Australian government document
NHMRC-Payment-of-participants-in-research-1.pdf
Curious why you think financial remuneration is not an option?

Interestin read on ethics of payment for participants. From what I read in 1.2 below it is perfectly allowable to offer payment as an incentive in cases where there is no benefit to participant or in specific populations that are difficult to recruit. Seems we have both of those options covered here?

1.2. Generally, payment of participants should be limited to reimbursement of documented
expenses and remuneration for time and inconvenience in order to minimise the
likelihood of a payment acting as an undue influence (or undue inducement). In cases
where the research offers little or no benefit to individuals (e.g. early phase clinical trials)
or where the research requires the participation of target populations that are difficult
to recruit, payment may be offered as an incentive to participation, as long as adequate
processes are in place to promote valid consent.