We have a great program (we call it the Leave Bank) for the past several years at least. Each person who elects to participate puts in x hours according to their leave benefits (I think 1 pay period's worth per year (~4-8 hr per ~2 wk per year)). We have no real max withdrawal, although the total deposits in the bank and the number of recipients can limit the total.
It is used for medical only, not extension/substitution for FMLA.
You have to be a member previously to be a recipient (no pre-existing conditions!).
When leave is exhausted from the leave bank, an email can be sent to all employees requesting leave donations (it may be anonymous or list the candidate's name).
Our fed agency has over 500 employees.