I truly hope we'll get major improvement to retainer interface when the PS3 support with it's gui limitations is dropped. What I would love to seecis possibility to open two retainer inventories at once for moving stuff between them.

As for collective item search, there actually is already. The slash command /isearch "partial name" works wonders, just remember the quotation marks in case you use multiple words for the search string. It lists you all items matching it that you own somewhere, and being itemlinks, you can then check who has them. It's a bit clunky I admit, but better than nothing and good to know.