There has been a lot of talk recently of the Bern Helmet and the amazing mod that Fixcraft have been selling a pre-made Bern helmet with face cage. These guys have really put together a great range of polo products from capped mallets to wheel covers and cages.
Although a face cage is purely a personal preference, for those that believe in it, this is a great option and possibly the best solution out there for bike polo. It is still essentially a dedicated cycling helmet, and the cage offers PLENTY of visibility and is less obtrusive that other solutions such as hockey or lacrosse cages.
They have recently added an instructional video above to show how easy and smooth the assembly is if you want to do it yourself.