The last thing you want to be doing is using polish. I know i sound evil (sorry) but who's bright idea was THAT? If you cant take them down then you need to clean each slat individually. It is a horrible horrible job, if i could rid the world of 2 things it would be furniture polish and horizontal blinds. You need to make sure that what ever you have quoted the client covers enough time to clean one blind at a time, i'd suggest that you did them all on different days on a rolling 2-week schedule as opposed to cleaning ALL of them once a fortnight (which may or may not be what you are doing now).
The office i cleaned had all the blinds fitted betweeen 2 pieces of glass used on the internal office windows, EXCEPT for the odd few which were just hanging to the sill on the external windows. I never cleaned those in all the time i was there (well over a year) i just waited to be told.........they never said a word. I hate these blinds so much. If asked to clean them in somones house i state that it comes at the cost of not doing other jobs as the blinds take sooooooooo long.
Best of luck
Stephen