I once had a tenant approach me on the issue. We told them we would do it ( at what i felt was a reasonable rate) they didnt take the offer. Their work did some type of 0% interest alone instead. Im sure there are 100 reasons why not to and lending laws etc but it was a 14 year year tenant and i think it would be hard for any ( conventional) lender to help them on this. I have seen a lot of people do upgrades their self as well over time.
@Ivan_ilych I did find out the city ( actually at the county level) did offer a program for low income tenants for energy efficient upgrades. Some tenants did do that and it worked out for them. Might be something to check as well.