I have had the same problem with oozing or dripping from the idle extruder, mostly the one with PLA in a 2 extruder setup. My first preference was to use PLA for raft and support, and ABS for object. Printing the raft in PLA is no problem and gives good adhesion from both the bed to raft and from raft to object. Once the object starts to print on top of the raft two problems arose, first the idle extruder is not receiving heating power so cools down...then when needed printer has to pause and heat extruder. Takes way to long to print while waiting for each extruder to reheat. Second problem is the PLA oozes and creates whiskers in the object which pretty much ruin it. My solution is to use PLA for raft and ABS for both object and support. Works quite well most of the time, just have to break away the support with needle nose pliers. The main thing lost is the ability to dissolve the PLA support in a ultrasonic tank.

My thought was it would take a way to close off the idle extruder with some type of closing iris but this is probably too complicated and expensive for the current market. Your idea of less volume to pool melted material at the end of the head sounds like a good one. If some extruders now have no ptfe tube at all as person responded that would appear better also.

I'm about to replace my second generation Rapman printer with a replacement. It still works mainly because almost 100% of the parts I remade in aluminum by hand so it would quit falling apart. Leaning towards Flashforge Dreamer.

Ole_blue