No time research this now, so has there ever been made a 28 hole front or rear drum brake hub? I know that Sachs and Sturmey-Archer have done a lot of drum brakes. Neither to my knowledge have such a hub in production currently. The reason for asking this is that I have 2010 version of Kona Band Wagon and I'd like to move to using drum brakes, but also retain the current rims that are 28 hole rims.
I've seen 36 and 48 hole brakes, but not 28. 28 strikes me as an unlikely number because hub gears/brakes are not usually made for especially lightweight bikes. Harris Cyclery only have 32 hole and 36 hole in their parts list, and I can't really make enough sense of the Shimano web site to really be clear on the options. Note that Shimano make "roller brakes" rather than drums, and those clip onto the side of the hub. Sturmey Archer drums all seem to be 36H.
If you are willing to really work at building the wheel you could cross-lace a 36H hub into a 28H rim, but you wouldn't get an especially strong wheel and it would take a lot of work (I expect you'd have to do it experimentally and that really means having a range of spokes sizes available on a "pay for what you use" basis).
Yes there have been 28 hole rear drum brake hub by STURMEY ARCHER. you can only get used, which means you have to rebuild it yourself. Plenty of them on ebay such as this one http://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&item=220912258562&ssPageName=ADME:B:BOC:US:1123
This depends on where you live, however, in the UK the only widely available hub brakes are the Shimano Nexus series. These are 36 hole and can be retro-fitted to an existing bike designed for rim brakes.
Given that you will be needing spokes as well as the hubs, consider getting some rims that will work with your hub brakes, i.e. not needing the surface for the rim-brake. Have fun building them up or pay the small fee that a professional wheel builder will charge.
Visit a good shop and go out on a bike that does have Shimano Nexus - try before you buy. They may not be what you are looking for - with performance benefit in the wet but not with the feel that you are used to.
Generally 36H is a good idea for wheels that have the stopping gubbins in the centre as the forces are different to that of a rim-brake wheel - you are decelerating the centre and exposing the spokes to more force.
By building a new set of wheels you will be able to keep the old wheels - or find a friend to take ownership of them - and not have a bunch of left over hubs 'n' spokes kicking around.