The nipples can be sealed without removing each spoke - you just need to apply a sealant that is not brittle and fills gaps. Here is a link to an article from a dual sport/adventure tourer type that sealed his rims.
http://adventure-motorcyclingh.com/2012/02/14/converting-spoked-rims-for-tube...The author comments on how the lack of the safety lip on the rims did cause him some leakage problems at the bead. A street tire may have this same issue on potholes.....not sure.
The rear tire on modern trials motorcycles are spoked and tubeless - they have a deep groove in the middle of the rim and a very soft pliable rubber strip lays in this groove and seales the spoke nipples. The valve assembly passes through this rubber strip to put all of the 4 psi into the tires. They do not use tubes as the tires are often compressed down onto the rim edges while riding and it would pinch the tubes and put holes in it.