Yes it is possible to trade a partial load of goods in an indian village. (Note I said 'possible', not 'easy'). I have just tested it and this is how it works. Load a galleon with an empty wagon and 450 guns and sail it into a coastal indian village. The 'Unload All' icon appears. Click this and your wagon unloads into the village. Then click on the guns you wish to sell (either 50 or 100) and the 'Load' icon appears. Click on that and the guns you wish to sell will be loaded into your wagon. Select your wagon and trade those guns to the indians. The remaining guns remain in your galleon. Load your wagon back onto the galleon and continue on to the next village - or load more guns onto your wagon and send it to an inland village if you wish.
This method only allows you to trade an individual hold of goods, in this case either 100 or 50, you cannot subdivide it further as you could in a colony, and you can only trade the 50-gun hold once. However it still opens up new trading options if that is the type of game you prefer.