i beg to differ. i think it may be in the quality of the doors being used. i have used hollow core doors for years and years and they have rarely warped. we use to create the whole design office out of them with doors, sawhorses and small drawer cubes.
i have two 7' x 2' ones (home depot luan doorskin) here in my office supported at each end by roll around drawer units and they have been flat and true for 10 years with lots of not so light crap piled all over them. im thinking they can take the weight of a small layout and then lift at like 25% in from the ends on the sides and i would doubt they would ever warp. moisture due to scenery could potentially cause problems with warping. painting them first (or buying pre-primed doors) might help this. or varnishing them first might also help. one issue with warping we found with the new thin jrm modules was even though the modules were sealed and totally flat and true after two coats of varnish, we put a coat of latex house paint on the top. this then warped most of the modules up a couple of mm. i was amazed that the latex shrinkage could do this differentially on one side! most of that has come out with time (i guess the paint relaxed or cracked a bit) but it was pretty shocking!
if warping was an issue you could just screw a couple strips of 1" L bracing along the two long sides.
like webskipper said simple dowel pegs should do your alignment. only trick is to drill the holes just right. to do this get yourself a doweling jig (like $15) or you could make a simple jig that would fit over the end of the module and locate the hole in the right place (you can make this so it reverses and it will come out exact).
we use pins like this on our new jrm modular layout and it works great. we just have the track break at the module interfaces and just clip the tracks together once the alignment is started by the pegs. you can use expansion tracks if you want, but be careful these can cause problems if put right near curves.
http://japanrailmodelers.org/photos/newlayoutmodules/index.htmlcheers
jeff