As far as I know, kill rights will expire 30 days from the point they’re earned, regardless of when or how many times they’re activated.
You can activate a kill right at any time to make someone suspect for 15 minutes. If they die, the kill right is removed. If they don’t, the kill right remains for however much more time has to pass to reach the 30 day limit. If you never use it, it still goes away in 30 days when its time is up.
Your entire transaction, from the initial loan and kill right generation to the repayment and/or harassment of the non-paying borrower, must fall within this 30 day span or your collateral/right to punish goes poof.
And, of course, this only affects people who do stuff in highsec with a character over -5 sec status they actually undock with.
I’m skeptical this could work, bar some exceptionally specific circumstances.