but client did not want to activate HU management for warehouse, which will increase the work of packing, repacking etc
If he made that choice, he will have to deal with the consequences of that choice. I does bring in extra work, but also brings more information and tracebility. If he didn't want the extra work (too much I know, having to repack in a special storage type is stupid), he will have to deal with the absence of TO number in pallet label, because there is no BAPI/BADI that does what you want, it goes against how SAP WM works.
He wants to have his cake and eat it. Can't have both ways.
PS: Contrary to your requirement, it is possible to abstract the pack/repack complexity from the user. I made a development for a client, that basically did all the (stupid) transfers in the background, making it easier for the user, using SAP APIs.