You are right in that it will only recieve dc power from the keep alive, the decoder will not be able to send or recieve commands from the control system while it is running on the keep alive over a dead spot.  The keep alive will keep the motor running and the light on using dc power until the engine is clear of the dead spot or the capacitors are drained, which ever comes first.  The ka1 and ka2's are not actually decoders they are banks of capacitors that you add to a dcc board.   Here is the the link to the tcs website that shows how you can use keep alive with a nscale engine.
http://www.tcsdcc.com/public_html/Customer_Content/Installation_Pictures/N_Scale/Atlas/C630/N_Atlas_C630_AMD-1.html

If they can stuff a decoder into that 44-tonner, i don't think they'll have a problem putting one in the tender lol.  I don't need to change the decoder itself, i just have to attach two wires from a small connector that connects to one of tcs' keep alive "decoders"( its actually a group of capacitors) that will be housed in a boxcar pulled behind the engine.  I will also be doing this to my model power 4-4-0 and 2-6-0, they will actually be converted to dcc with a decoder install.

They do look good, but thats not why i want the engines.  I want them because they already have dcc.  I have also been in contact with tcs about using there keep alive decorders with these engines, and if it works then I won't be pulling my hair out whenever it looses power, the only problem with small n-scale steam, and will thoroughly enjoy running them.

does anybody know if the new 2-6-0's are going to have oversized tenders like the 4-6-0 did?  if they have the appropriate small tender with dcc a standing ovation will be required for bachmann.  also can't wait to get my hands on an s4.